In Fallout 3 you gain a perk every level, starting at level 2, which you’ll hit as soon as you get through the prologue. Perks are seperated by thier level prerequisite, with every even level opening up more perks. Prerequisites for perks may also include S.P.E.C.I.A.L attributes or skill points. (For example, the Entomologist perk requires that you be level 4, have an Intelligence score of four, and a Science skill score of 40.)

Perks have a variety of effects… sometimes they give you a boost to various skills (these are the worst of the perks, and should generally be shunned), others replicate certain Attribute-driven bonuses (high melee damage, carrying capacity, or Critical Hit Chance)… and some are just unique, wildly affecting combat or even opening up new quest or gameplay options.