The Androids were created by Dr.Gero in his lab.There are 8 androids total including Cell.First 19 and 10 appear in the Android Saga and get crushed. 19,who looks like a fat siamese circus freak, is killed by Vegeta and 20 who is actually Dr.Gero is getting crushed by Piccolo and thren runs away to his secret lab to activate 16,17 and 18 who kill dr.gero right after.Soon Cell comes and kills 17 and abosrbs 18's energy to be complete. Androids 13,14, and 15 come later in DBZ and Vegeta kills 2 of them and Trunks kills one.As most of you know Cell is killed by Gohan.Also 18 marries Krillin which I find very odd.And in DBGT 17 is revived to become Super 17 who Goku kills with a 10x Kamehameha.