#16

The first android to be created by Dr. Gero, 16 was pretty powerful in his own right.  But he was different from the other androids.  He was the more peaceful type, who didn't really enjoy the fight too much and quiet most of the time.      Later in the Cell series, 16 get's injured trying to fight Cell in his first stage.  He sits out for the rest of the series and trys to give advice to the Z warriors.  When Cell was hunting 18 for completion, 16 stayed with her and tried to asure her not to worry, and even tried to defend her when Cell found her.  16 is later destroyed by Cell when Gohan is fighting Cell.

#17

Android 17 was created with 16 and 18.   He along with 18 turned on Dr. Gero and killed him.  After fighting the Z warriors they left to do search for Goku.  He fights with Piccolo in a dead even match, but Cell shows up in the middle of the fight and Piccolo and 17 team up to fight him.      In an alternate timeline, he and 18 rampage across the Earth and kill all Z warrior except for Trunks, who laters kills both 17 and 18.      In DBGT, 17 becomes a main villain when he melds with an evil 17, made by Dr. Gero in hell, and becomes Super 17.  He is destroyed by Goku.

#18

Android 18 was created by Dr. Gero with androids 16 and 17.  She, like 17, showed no sympathy for anyone and basically killed numerous humans.  Later in the Cell Game, 18 was absorbed by Cell to meet his completion.  After Cell was struck by Gohan, however, 18 was spat back out.  After the battle with Cell ended, the Z warriors wondered what they would do with 18.  Everyone pretty much agreed to do away with her except for Krillin, who had fallen in love with her.      Later, 18 married Krillin and had a baby girl with him.  She was named Marron.  18 eventually faded out in the Buu series and didn't serve too much of a role at all.      In an alternate timeline, as said before, she rampaged the Earth and was killed by Trunks.      In DBGT, when 17 becomes Super 17, she fights her twin brother.