Powergaming is a particular way of playing a role-playing game in which the emphasis is developing a powerful player character. Powergamers are frequently contrasted to role-players, who emphasize character, personality, and motivation.
This will sometimes result in characters which seem inconsistent with other aspects of the game world. For example, a Dungeons & Dragons sorcerer might take a single level of the paladin class, when paladins were traditionally seen as being devoted to their lifestlye, and not something you could train in briefly.
In many cases the games themselves, especially computer role-playing games, have evolved in ways that aid powergamers. Neverwinter Nights is one example of a CRPG that permits players to create many different class combinations that are inconsistent with earlier versions of Dungeons & Dragons. Players often describe the process of playing for the sake of advancement as powerleveling.
On MUD and MUCK systems that emphasize social interaction as well as role-playing, a powergamer is a player who tries to force others, by his actions, to participate in role-playing they don't want to engage in. For instance, a player who unilaterally describes his character as doing something with (or to) another character which would usually require the other's playing along -- such as having a fight or a sexual encounter -- is considered to be powergaming. Powergaming in this sense is regarded as bad style at best, or abusive at worst.
