Android 18 (人造人間１８号 Jinzōningen Jū Hachi-Gō, lit. "Artificial Human No. 18") was originally a normal human named Lazuli (ラズリ Razuri), but was transformed by Dr. Gero along with her twin brother Lapis into Cyborgs (incorrectly called Androids in the dub). Her programmed purpose is to destroy Goku with Android 17. However they are able to defy their programming and destroy their creator, and while pursuing Goku at first, they eventually abandon their programming completely.
During the Cell Saga she is confronted by Krillin who has a device Bulma created which would effectively destroy her thus making her useless to Cell. Krillin, however, is unable to push the button and she is eventually absorbed by Cell to complete his Perfect Form. This is however the foundation for the love that eventually blossoms between 18 and Krillin. They eventually get married and have a daughter together named Marron. They all live together at Kame House (Master Roshi's home). Krillin and the others used the Dragon Balls to wish for Android 18 to be changed back into a human being, however Shenron explains that it is beyond his power to grant the wish and 18 is forced to stay the way she is. They do however manage to remove the bomb that was inside her chest.
Her hair never grows (and despite that, it had been cut in Dragon Ball GT) and she is also immortal which means she never ages (lucky Krillin!). She apparently has no memories prior to being an Android. During the struggle against Super Android 17 in Dragon Ball GT, 18 threatens to detonate the bomb in her chest, knowing that Super 17 didn't know it had been removed years before.