Cell has 3 different forms. In his first form he uses his tail to absorb people. He changed form by absorbing androids #17 and #18. This made him become Perfect Cell. During the battle with Perfect Cell, Cell released 7 minions called Cell Jrs to attack the Z warriors. They were all killed by Gohan.

During the Cell Game, Gohan made Cell spit out #18 and he changed back into his second form. Then Cell decided to self destruct, so Goku teleported him to King Kaio's planet. This killed Goku and everyone on Kaio's planet, but one of Cell's cells survived so he regenerated into Perfect Cell again. In the end he was killed in a great Kamehameha duel between him and Gohan.

Here is a description of how Cell got to the present time and what happened in each of the timelines etc:

In Trunks' future Gero considered the project a failure and left it alone. However the machine was still working and Cell was being completed anyway. #17 and #18 killed Gero and blew up the lab. They then went to terrorise the Earth. Cell was not destroyed however and was left to be finished, which would take 3 years. Future Trunks went back in time and warned the Z warriors about the androids before returning to the future. This created a second timeline. Then 3 years later Trunks went to the present timeline to help fight the androids.

However, 3 years after Trunks' future (aka Cell's future), Cell was completed. He killed Trunks and used his time capsule to travel to the present time. This created a third timeline. Now everyone was in the present timeline. The Z warriors destroyed the Cell which was developing in Gero's lab. Cell from Cell's future absorbed #17 and #18 and became Perfect Cell. He was eventually killed in the Cell Game.

Then Future Trunks went back to his time and first killed #17 and #18 so Cell couldn't become perfect. He then killed the 1st form Cell from his time which had just been completed.