[The Z]

Perks in Fallout were special bonuses that you could choose every 3 levels (4 if you had the Skilled trait). An example might be that you choose the Magnetic Personality perk, which means you can have one more NPC in your party. Or there's the sharpshooter perk which increases either damage or ability to hit (I can't remember which) with guns. The perks you can choose are better at higher levels (they don't improve though, it's a onetime bonus sort of thing); i.e, in Fallout 2 once your levels were high enough, some perks you could choose permanently increased your ability scores. It's a pretty cool system actually.