Alternate Timeline

Time map2-1- — The four alternate timelines

Several alternate timelines are shown to exist in the Dragon Ball manga and the Dragon Ball Z anime series. Shown below are the specific timelines and their specific events. A number beside a character's name indicates which dimension (what timeline) they are from; if there is no number beside the character's name, that means that they are from the dimension (timeline) the description is under.

Time Machine generated timelines

The alternate timelines were all created and linked due to the use of a Time Machine. Because the Future Android 17 and Future Android 18 killed the Z Fighters and most of the potential heroes of Earth, Future Bulma created a Time Machine to send her son Future Trunks to the past to prevent Goku from dying of a Heart Virus, to warn the Z Fighters about the future android threat, and to find a way to stop the androids in the future world (either a way to deactivate the androids or a way to destroy them, maybe even taking Goku to Future Trunks' timeline).

It should be noted that, in the universe of Dragon Ball, one does not have to directly interfere with a timeline to disrupt it. Simply going back in time causes disruptions to the timeline as various random changes occur. For example, Androids 19 and 20 came first, despite no explanation occurring for why they would exist, and Androids 17 and 18 are also inexplicably stronger than Future Trunks remembers. Therefore, even if a time traveler follows a "look but don't touch" policy, they risk changing history.

Timeline 1 is where the main story takes place, and Future Trunks and Cell arrive there from their respective future. Timeline 2 is the original timeline from where Future Trunks originates. Timeline 3 is where Cell originates from and goes back in time by killing Trunks and stealing his Time Machine; this timeline is briefly seen. Timeline 4 is never seen, but it is the timeline in which the Trunks from Cell's original timeline (3) goes back in time to find a mean to destroy the androids.

Timeline 1: Main

Gohan-blast — Gohan kills Cell in the main timeline

This is the main timeline in which the Dragon Ball series takes place and ultimately leads to the Majin Buu story arc. It is possible that timeline 4 also leads to a similar arc, one where Goku never died in the first place. There are two time/dimensional travelers who interfere with this timeline: Future Trunks from timeline 2 and Cell from timeline 3.

Cell³'s travel to the past and Future Trunks'² trip to the past create this timeline, which "replaces" timeline 4.

Cell³ arrives from the future from the Time Machine he stole from Future Trunks³ in an egg form because he could not fit. He hatches and burrows underground to develop.
Frieza and King Cold invade Earth to get revenge on Goku, and to also destroy the planet. They are killed by Future Trunks² who appears from the future. Future Trunks² then meets Goku when he arrives on the planet and gives him the Heart Medicine, from 20 years in the future; he also warns the Z Fighters about the Red Ribbon Androids who will arrive in three years, before returning to his timeline.
In the next three years, the Z Fighters go into intense training, and Trunks¹ is born to Vegeta and Bulma as their first child.
Dr. Gero (aka Android 20) and Android 19 battle the Z Fighters. Goku, who did not take Trunks' medicine/antidote, becomes sick during his fight with Android 19. Vegeta arrives and takes over, destroying 19, and Goku is taken home and given his medicine. Dr. Gero flees.
Future Trunks² arrives back to this timeline and finds the destroyed Android 19, a robot he has never seen before. He begins to realize that this timeline is starting to branch off considerably from his.
Dr. Gero escapes back to his lab and awakens Android 17 and Android 18 who then proceed to kill him. They go on to activate Android 16.
The Z Fighters are defeated by 17 and 18, but none are killed.
Future Trunks³' Time Machine from timeline 3 (used by Cell³) is found, along with Cell's shed skin.
Cell³, now fully grown, starts razing cities and absorbing the people to raise his power.
Piccolo fuses with Kami and battles against Cell³. Cell tells Piccolo about who he is, why he is here, and how he got to this timeline. Cell escapes after Future Trunks² and Krillin arrive on the scene to aid Piccolo.
Piccolo hunts for Cell³ while Future Trunks² and Krillin go to Gero's underground lab to destroy Present Cell¹ while he is still gestating. While there, Future Trunks² finds blueprints for the androids which he gives to Krillin to take to Bulma.
To become stronger, Vegeta and Future Trunks² train in the Hyperbolic Time Chamber.
Piccolo, unable to find Cell, is confronted by the androids. He decides to attempt to destroy them in order to prevent Cell from absorbing them and becoming stronger.
During Piccolo's battle against Android 17, Bulma contacts Krillin telling him that she is created an emergency shutdown device based on Gero's blueprints. It has a range of 10 meters so Krillin rushes to intercept her and obtain the control device to deactivate and destroy the androids. Cell³ detects Piccolo's raised ki and deduces that he is battling an android; Cell rushes to the scene to absorb the androids and complete his development.
Cell³ interrupts Piccolo and 17's fight, quickly dispatching Piccolo and nearly absorbing 17 before Android 16 intervenes. Cell and 16 fight relatively evenly, but in the end Cell manages a surprise attack that allows him to absorb 17 and become Semi-Perfect Cell. Cell proceeds to easily deal critical damage to Android 16 and is going to absorb 18 but is halted temporarily by Tien Shinhan, allowing 16 and 18 to escape. Goku quickly arrives on the scene and rescues Piccolo and Tien before retreating again.
Vegeta and Future Trunks² emerge from the Hyperbolic Time Chamber, letting Goku and Gohan enter. Vegeta battles against Cell and is clearly dominant in the fight. While they are fighting Krillin approaches Android 18 with the shutdown device, but is captivated by the beautiful android and destroys the remote. Vegeta quickly becomes arrogant with his power and gives Cell leave to absorb 18 to attain his perfect form.
Cell finds 18 and finally absorbs her, becoming Perfect Cell; he then goes on to defeat Vegeta and Trunks. Cell decides to hold a tournament, the Cell Games, in ten days to test his new abilities.
Goku and Gohan emerge from the Hyperbolic Time Chamber. Trunks, Vegeta and Piccolo all use the room before the tournament starts. Goku recruits Dende to serve as the Earth's new guardian and reactivate the Dragon Balls.
The Cell Games start and Goku fights with Perfect Cell. Goku eventually forfeits the match and chooses his son Gohan fight instead.
Gohan fights Perfect Cell and, after eventually being provoked into transforming into the Super Saiyan 2 stage, delivers a fierce set of hits to Cell, causing him to regurgitate Android 18 and transform back into his Semi-Perfect state.
Semi-Perfect Cell attempts to destroy the Earth by self-destruction, but Goku sacrifices himself to teleport Semi-Perfect Cell from the Earth to King Kai's planet. Cell explodes, killing Goku and King Kai. Cell's regenerative organ miraculously remains intact, allowing him to regrow himself. Cell returns to Earth, stronger than ever and kills Future Trunks², but is defeated for good by Gohan.
Future Trunks² is revived by a wish made to Shenron, and returns to his timeline.
The rest of Dragon Ball Z happens.

Timeline 2: Future Trunks

This is the timeline belonging to Future Trunks and depicted in a few chapters of the manga. This is the timeline in which any version of Future Trunks did not visit the present timeline. The first part of this timeline, common to timeline 3, is also depicted in the TV special Dragon Ball Z: The History of Trunks. In this one, Future Trunks goes back in time to the main timeline (1) and returns with enough power to kill both the androids and Future Cell. This timeline can be seen as the "improved" version of timeline 3, which was the "first version" of the timeline for Future Trunks³. Thanks to Cell³'s trip to the past in timeline 3, this timeline and timeline 1 were created. In order to defeat Cell³ in timeline 1, Future Trunks trained in the Hyperbolic Time Chamber and increased his power. Thanks to his new power, he was able to destroy the Cell from timeline 2 when he came back.

FTrunksTime(UD) — Future Trunks' timeline before he travels to the past

Frieza and King Cold invade Earth and are defeated by Goku.
Trunks is born, Goku dies from Heart Virus.
Dr. Gero activates Android 17 and Android 18. They kill him and then proceed to kill all of the other Z Fighters except for Gohan and the Dragon Balls permanently disappear, because of Piccolo's death. Gohan becomes a Super Saiyan.
Over the course of thirteen years Gohan challenges the androids alone but can not defeat them.
Gohan trains Trunks, but is eventually killed by the androids. Trunks attempts to avenge Gohan but is badly beaten.
Bulma builds a Time Machine to send Trunks to the past with the hopes of finding a way to destroy the androids. Trunks takes medicine to cure Goku's¹ heart condition with him.
Trunks goes back in time to timeline 1. He returns after killing Frieza¹, giving Goku the medicine, and warning the Z Fighters about the androids.
Trunks goes back in time again to help the Z Fighters in timeline 1 to combat the androids, and later Cell³.

New city — The city rebuilt after the destruction of 18 and 17

Trunks returns from timeline 1 more powerful as a result of his battles and training while there. He quickly finds and kills Androids 17 and 18 with little effort.
Trunks kills the newly emerged Cell², knowing about Cell's plan to kill him and go back in time (see timeline 3).
Earth slowly rebuilds its cities as peace comes to its inhabitants.
This timeline is continued in the PSP game Dragon Ball Z: Shin Budokai - Another Road.

Timeline 3: Cell

CellGettingOut — Cell just before he escapes Dr. Gero's underground laboratory, in his timeline

This timeline follows a similar path to timeline 2, it notably changes on Future Trunks' second return from the past as in this timeline, Future Trunks returned with the android blueprints from timeline 4, not the massive power increase from timeline 1. The androids were destroyed, but Trunks was not powerful enough to combat Cell. Cell killed Future Trunks and used his Time Machine to go back in time to find the androids, creating timelines 1 and 2.

This can be seen as the "original" timeline, as no time traveller arrives to this timeline from a different future. It is also the darkest timeline, as even Future Trunks is deceased. This timeline can be seen as the first "version" of timeline 2.

Frieza and King Cold invade Earth and are defeated by Goku.
Trunks is born, Goku dies from heart disease.
Dr. Gero activates Android 17 and Android 18. They kill him and then proceed to kill all of the other Z Fighters except for Gohan. Gohan becomes a Super Saiyan.
Over a course of thirteen years, Gohan challenges the androids alone, but he can not defeat them.
Gohan trains Trunks, but is eventually killed by the androids. Trunks attempts to avenge Gohan but is badly beaten.
Bulma builds a Time Machine to send Trunks to the past with the hopes of finding a way to destroy the androids. Trunks takes medicine to cure Goku's⁴ heart condition with him.
Trunks goes back in time to timeline 4 (which is created because of this trip). He returns after killing Frieza⁴, giving Goku⁴ the medicine, and warning the Z Fighters⁴ about the Red Ribbon Androids.
Trunks goes back in time again to help the Z Fighters⁴ in timeline 4 to combat the androids.

ImperfectkillsFutureTrunks — Cell strangles and kills Trunks in his timeline

Trunks returns from the past and destroys the androids after using their emergency stop circuit.
Cell emerges and, realizing the androids are gone, decides to use Trunks' Time Machine to go back in time and find the androids in the past. Cell ambushes and kills Trunks, gets the Time Machine and goes to Age 763¹. This eventually creates timeline 1 and 2. In timeline 1, Cell is destroyed by Super Saiyan 2 Gohan¹ with a Father-Son Kamehameha.
Even though Trunks died in this timeline, both Cell and the androids are gone, so Earth presumably starts to rebuild itself.

Timeline 4: Unseen

Little information is given about this timeline, except that the Cell Games happen, but Future Trunks is absent. It is possible that this timeline branched off after Future Trunks'³ second visit, making this a timeline where Future Trunks never visited for the second time and thus the Cell Games happened without him.

Cell arrives from the future from the Time Machine he stole from Future Trunks³ in an egg form because he could not fit. He hatches and burrows underground to develop.
Frieza and King Cold, invade Earth to get revenge on Goku. They are killed by Future Trunks³ who appears from the future. Future Trunks³ meets Goku when he arrives on the planet and gives him the heart medicine; he also warns the Z Fighters about the Red Ribbon Androids who will arrive in three years, before returning to his future.
Dr. Gero (aka Android 20) and Android 19 battle the Z Fighters. Goku, who did not take Trunks' medicine/antidote, becomes sick during his fight with Android 19. Vegeta arrives and takes over, destroying 19, and Goku is taken home and given his medicine. Dr. Gero flees.
Future Trunks³ arrives back to this timeline and finds the destroyed Android 19, a robot he is never seen before. He begins to realize that this timeline is starting to branch off considerably from his.
Dr. Gero escapes back to his lab and awakens Androids 17 and 18, who then proceed to kill him. They go on to activate Android 16.
The Z Fighters are defeated by 17 and 18, but none are killed.
The Z Fighters find the blueprints of the androids and use it to create a remote control, allowing them to deactivate the androids in this timeline.
Future Trunks³ leaves back to timeline 3 to deactivate the future androids.
Because Future Trunks³ was killed by Future Cell, the Cell Games happen without him coming back. It is unknown what happens after.

Character summary

Character(s)	Timeline
Character(s)	1	2	3	4
Frieza and King Cold	Killed by Future Trunks²	Killed by Goku		Killed by Future Trunks³
Goku	See Goku	Died from heart virus		Likely lived thanks to antidote from Trunks³
Trunks	See Trunks	See Future Trunks	Killed by Cell	Likely lived
Other Z Fighters	See respective biographies	Killed by androids		Likely lived since Goku lived and the Androids in that timeline did not seek to kill the rest of the Z Fighters, like in the main timeline, and because the Androids were deactivated
Androids 17 and 18	Absorbed by Cell³ but eventually freed and reformed; see Android 17 and Android 18	Killed by Future Trunks in combat	Disabled and destroyed with device based on blueprints obtained by Trunks in timeline 4	Likely destroyed by device made based on blueprints
Cell	Killed as a fetus by Future Trunks² and Krillin	Killed by Trunks² in failed ambush	Killed by Gohan¹ in timeline 1	Likely killed by Future Trunks³ and Krillin⁴ as a fetus like in timeline 1, as the android's blueprints were next to him

Trivia

Cell³ traveled back in time to Age 763, according to him, because that was the year that was programmed on the machine. But if Future Trunks³ story is similar to Future Trunks² story, he was about to use the machine to tell the Z-Fighters from storyline #4 that he had defeated the androids in his universe. However, there is no reason for Future Trunks³ to go to Age 763 to do that; nobody knew him at that time, since his first trip was to Age 764. (This might only be explained by the inaccuaracy of the time machine.)

References

Dragon Ball volumes 28-35
Daizenshuu 7