18 is a cybernetic human, modified by the scientist named Dr. Gero. She is often referred to in the original Japanese as a jinzo-'ningen, which literally translates as "artificial human" (jinzo--ningen), thus "Android, robot". However, she is not entirely an android because, in a conversation with 16, she refers to both herself and her "twin brother" 17 as having been "constructed from a human base". It is implied in the manga, and the original Japanese version of the anime, that they were originally runaways heavily fitted with cybernetics. In the Japanese version of the anime, 18 states that she and 17 were abducted by Dr. Gero, and have hated him for what he did to them, ruining Semi-Perfect Cell's attempt to trick her with 17's voice. However, she is able to biologically function as a human does and has a working reproductive system, given the fact that she gives birth to Marron right before the Majin Buu Saga.
18's original human name is never revealed, and following the defeat of Super Perfect Cell and the removal of her self destruct mechanism she is referred to as simply "18". She is at first noted for her cool and confident yet sarcastic attitude when she first joins the Z Fighters, although later on she loosens up a bit and even smiles a lot.
Other than the destruction of Goku, it is never explained why 17 and 18 were created, but it has been suggested that 17 and 18 were tools to be used by Dr. Gero's ultimate creation named Cell. Imperfect Cell was supposed to absorb them into his fully biological body and become the "perfect" warrior. He achieved this, absorbing both of the cyborg twins, 17 first.