Prostitutes (aka Hookers) have been in all the Grand Theft Auto games since GTA III. They are only out on the streets at nighttime, and to get their services you have to drive close to them and honk your horn. Then you have to go someplace secluded and you start "interacting" with them.
Prostitutes can get your health up past 100%, but it costs you money. However, you can kill the hooker and take your money back when you're finished. Such acts are commonly used in referencing how "bad" the GTA games are.
In GTA San Andreas, you could do "Pimping" missions, where, once you reached the max level, hookers would pay YOU for having sex with them.