As per 3.5 handbook:
At 1st level, a ranger may select a type of creature as a favored enemy. Due to his extensive study of his foes and training in the proper techniques for combating them, the ranger gains a +1 bonus to Bluff, Listen, Sense Motive, Spot and wilderness Lore checks when using these skills against this type of creature. Likewise he gets the same bonus to damage rolls......................