Jinzouningen 18-gou is a cybernetically enhanced human, modified by Dr. Gero. In the original Japanese, she is referred to as a jinzouningen, which translates to "artificial human." She is not purely an android; she describes herself and her twin brother as having been "constructed from a human base."
In Future Trunks' timeline, No.18 is a mass murderer. After Cell is defeated, Kuririn uses the Dragon Balls to wish for No.17 and No.18 to be turned into humans. However, Shenlong cannot fulfill this wish due to the immense power difference between him and the duo. Instead, he removes their self-destruct devices. Kuririn then pursues No.18, and they eventually marry and have a daughter named Marron.
To clarify the common misconception that No.18 is a pure android and thus incapable of having a child, Kuririn explains to Son Goku during the World Tournament Saga that she was originally human but was "remodeled a little bit" by Dr. Gero.
(Source: Wikipedia)
