Have you ever wonder how Trunks was dead and alive at the same time? Well, you have seen how the bio-android, Cell was able to steal Trunks' time machine and travelled to the past. If you were confused of how it happened in the first place, then I'll try to make it simple by explaining two timelines. Maybe Cell have explained it for you, but let's go ahead with the theories here.
Timeline A (Original Timeline)
Everything that happened from Goku having a heart disease and the androids destroying the rest of the fighters, Gohan and Trunks were left alone defending Earth from the androids. Gohan gets killed and Trunks became a Super Saiyan to battle the androids but he was left unable to defeat them. Trunks decided to go back in time when his mother, Bulma created a time machine. Unfortunately, the law of physics doesn't apply here because if Trunks had altered the past, it woudn't affect his future. Don't ask me why. It supposed to happen but I guess the DBZ creators have used their own rule of time travel. You only create an alternate reality once you have arrived in the past. Trunks went back in time and saves Goku from the heart disease. Later, he explains his dark future to the rest of the Z fighters and find a way to defeat the androids. Trunks travels back to the future and successfully deactivated the androids with some kind of control device. At this time, Imperfect Cell was born and later found out that the androids were deactivated because he needed to absorb them to become stronger. When he learns of the time machine, he instantly kills Trunks before he tells the good news to his new friends and fighters about the android deactivation. Cell evolves back into his larva stage and placed himself in the time machine and then travels further to the past even before Trunks arrived to kill Frieza and his henchmen. Since Cell is the second time traveller to change the past, this timeline A later becomes Timeline B.
Trunks arrived in this timeline and successfully saved Goku's life from a heart disease which basically happened the same as Timeline A. How was Trunks alive? No, he's not alive. Let's say that this Trunks is called Trunks 2. We know that Trunks 1 was killed by the Imperfect Cell. Trunks 2 has to exist because Trunks 1 came to the past already for the first time. In this timeline, Trunks 2 discovered another time machine which was used by Trunks 1. If you think about it, the Imperfect Cell have arrived from Timeline A (the original timeline) to wait for the incoming Androids because he knows that they may still be alive in the past. However, Trunks 2 came from the same future as Trunks 1. This future is not considered Timeline A because Cell have arrived in the past already before Trunks 1 did. If Trunks 1 hadn't arrived in the past in the first place, Trunks 1 would have been dead and there is no way that Trunks 2 can go back and save Goku from the heart disease and help the fighters to battle the androids. Which means Trunks 1 is the same as Trunks 2 but a different version. Because up to this point, Trunks 2 trains himself in the past to become a better fighter. Trunks 2 trains with his father, Vegeta to battle Cell after it aborbed the two androids. During the battle, Cell explains to Trunks 2 of how he was able to travel back in time in the first place. But Trunks 2 wasn't powerful enough to defeat Cell in its third and full stage. When young Gohan (Super Saiyan 2) successfully destroyed Cell, the powerful Trunks 2 goes back to his future and successfully destroyed the androids in one blow without the control device. Later, Imperfect Cell 2 finds out that the androids were destroyed. Before Imperfect Cell 2 takes the same action from Timeline A, Trunks 2 easily defeated the Imperfect Cell 2. Just imagine, if Trunks 2 hadn't trained in the past and became powerful, all the events in the Timeline A would have been done repeatedly. As a result, Imperfect Cell 2 would still go back in time earlier before Trunks 3 arrived in the past and hence we have Timeline C.
I know time travel makes your mind boggle sometimes. But what do you think? Does this make sense?