Other forms: thieves
A thief is someone who steals something. A thief can be anything from a cyberspace criminal mastermind who steals credit card information around the world, to the petty thief who picks people's pockets on the subway.
Some famous thieves (notice the -v- in the plural) include outlaws of the Old West, like Butch Cassidy and Jesse James, and those notorious bank robbers of the 1930s with the memorable names — "Baby Face" Nelson, "Pretty Boy" Floyd, Bonnie and Clyde, Ma Barker, "Machine Gun" Kelly, to name a few. They were so bold (and so brutal) that their activities became a major impetus for the beefing up of the FBI, under J. Edgar Hoover.