Thief/Fallout 3

Thief is a perk in Fallout 2, Fallout Tactics, Fallout 3 and Lionheart.

Fallout 2 and Fallout Tactics
+10%  to your Sneak, Lockpick, Steal, and Traps skills. As with all skill point perks, this one is useless.

Fallout 3
+5 points to both the Sneak and Lockpick skills. With three ranks, the player gets a +15 bonus to Sneak and Lockpick.

Lionheart
+10% to your Lockpick, Find Traps/Secret Doors and Sneak skills.