Android 16: His sole mission is to seek out Goku and kill him. Though after being with 17 and 18 for awhile, he sees what it's like to live, and changes. He chooses to leave Goku be, and fights along side the good guys while he lasts.
Android 17: Android 17 was once human, but Dr Gero did a little re-modifying, and thus created andoid 17. He is extremely powerful, and another ego-man. He thinks he's a big tough guy, and toys with Piccolo while they fought, but his toying around lead Cell right to him, and his end.
Android 18: This deadly diva is tougher than she looks. She's one tough cookie, as Vegeta found out the hard way. Though her and 17 were once human, they were modified and made into highly destructive androids, bent on destroying Goku and the Z team.
Android 19: This guy is just one crazy android. As if his looks weren't funny enough, sit down and listen to the episodes where he and Goku fight when you have free time. Listen to the amount of odd and funny noises that come from him when Goku's is pumeling him. It sounds more like he's getting a hernia while taking a dump and having a brain hemorhage at the same time. This truely is something to make a sound clip out of. Now then, 19 was made after 17 and 18, but if far weaker. He is an energy absorbing model, which make him easier to control, but he has a problem of running low on battery power. He is destroyed very easily by Vegeta.
Dr Gero/Android 20: This old coot should just roll over and die. He's been alive for way to long. Ok, ok, he's only like 45 or somethin, but he looks like you could grab quite a bit of loose skin there. Ok, on to the bio. Dr Gero has hated Goku ever since he defeated his Red Ribbon army way back in the day. He creates the Androids with the intention of destroying Goku, but to no avail. The androids kill him instead, and never even lay a hand on Goku(except Cell, but i'm not talking about him right now).