Android 16 is a android created by Dr. Gero to kill Goku but the doctor Gerodidn't want to activate it thinking that this it would not be useful because he "loved" to the nature and from time to time it disobeyed him.
This android was activated by the androids 17 and 18. They killed Dr. Geroand decided to activate at it. Android 16 was never activated in the time ofFuture Trunks.
Android 16 didn't want to fight with the warring Z fighters because it was programmed to kill to only Goku and not them, but when Cell appeared, 16's intent changed to help its siblings cut the monster's tail. But Cell's tail regenerates thanks to the Piccoro cells, and Cell absorbs Android 17. Android 16 then becomes intent to protect Android 18, but failed.
Android 16 gets destroyed, all except its head. The head then rotates asksMr. Satan to throw it where Cell and Gohan are. Mr. Satan obeys, but Cellsquashes the head destroying it completely. This act was what caused the anger of Gohan to increase to the point that he transforms in Super Saiyajin 2.
Andriod 16 didn't appear in the series again because it could no longer be repaired and when not possessing a soul to be a robot they could not revive him with the Dragon Balls.