The "Thief" series of video games are "first-person action RPG" games wherein the player takes the role of Garrett, a thief in a mythical time somewhat like the middle ages. Garrett starts out each one as an amoral character, but ends up saving the world in each.
The main concept behind Thief was to turn the first-person shooter idea on its head... everyone plays first person games with the idea of killing enemy after enemy in a RAMBO-ish style until bored or until the game is beaten. But Thief takes a different approach; the main tactic of the game is to avoid fights, and not to kill.
The game is sometimes described as either a "First person sneaker" or "First-person looter" to describe the difference.
Through careful use of sound effects and a very involving plot line, the player is drawn in to the milieu of the game.
Originally published by Looking Glass Studios. However, they've gone out of business, and so the third game of the series is being done by developers at ION Storm Austin (the team includes many ex-Looking Glass employees) and will probably be published by Eidos Interactive.