This page focuses on Trunks from the alternate future Dragon Ball Z timeline, to read about the Trunks from the main timeline, check out the Trunks page.
Trunks is the son of Vegeta and Bulma, first appearing early in the Android Saga. When Trunks first appears, he is met by Frieza's small and very weak army. After a short taunting and a monologue, Trunks turns super Saiyan just before Frieza and his father King Cold attack Trunks and are quickly destroyed by his unique sword and staggering power level. Coming from the future, he warns Goku of both a fatal heart disease that he will contract, and of a pair of androids ( Android #17 and #18) that kill all of the Z-Fighters in his time. After doing so, he then returns to his timeline for 3 years, traveling back to the normal timeline once again, this time to help defeat the androids he had warned Goku about, who consequently became much stronger due to his interference and telling the Z Fighters to train and become much stronger than they were in his own timeline. Goku becomes aware of the need to use the Hyperbolic Time Chamber and offers the use of it to Vegeta and Trunks who would train in it first. During this time, Piccolo fails to defeat Android 17 and is overwhelmed by Cell who eventually absorbs #17 to become imperfect Cell. Trunks and Vegeta fight an Imperfect Cell but fail to prevent Cell from absorbing #18 and becoming complete, because of Vegeta's interference, he then fights Cell after Vegeta fails to kill Cell. After being defeated by Cell, Trunks then informs everyone of the 'Cell Games' that Cell announced. Future Trunks once again trains in the Hyperbolic Time Chamber but during the events of the Cell Games, Trunks is impaled by an attack from Super Perfect Cell that subsequently kills him. After Cell's eventual defeat at the Cell Games, Trunks is revived by Shenron and then returns to his own time. Upon his return to his timeline, he finds the location of Androids #17 and #18 and easily defeats the two, who are unaware of his significant boost in power. Three years after Trunks defeats the androids of his timeline he sets up his time machine to go back to the past to tell the Z fighters of his accomplishment but in the process ends up fighting the Cell of his timeline who is looking for the Androids, whom are now destroyed. Trunks easily outclasses this Cell and defeats him before he can use Kamehameha powerful enough to destroy Earth. Trunks finally is able to return to the past and tell the Z fighters of the peace he has finally earned in his timeline, which marks his last appearance in Dragon Ball Z.
Alternatively: A different future Trunks was able to help defeat the Androids in the original timeline and avoids the entirety of Cell in that era. He is much weaker than the Future Trunks mentioned above due to not having had to train in the Hyperbolic Time Chamber. In his timeline he is able to disable the Androids and on his return to the past to inform the main timelines Z Fighters. He is killed off by Cell who steals his Time Machine in order to go back to a time where the Androids do exist. This would explain an encounter Future Trunks has in which he finds a very aged version of his own time machine, of which there is only supposed to be one of.