There are multiple perks in Fallout 3 and the player can select one at each level up. Perks have a variety of benefits, from improving prowess with melee weapons to allowing the player to eat corpses for health regeneration.

In Fallout 3, players can select one new Perk each level, beginning at level 2. Some perks have stat or level requirements; others have multiple ranks that can be increased at each level up. For example, the Iron Fist perk is unlocked at level 4, requires a Charisma of 4 and has three ranks. As players level up, the requirements for perks will go up, and the effects of those higher level perks will make more of a difference.

In addition to regular perks that are unlocked as the player levels up, there are also perks given as quest rewards. These include Ant Might and Power Armor Training.Fallout Wikia: Fallout 3 perks