Gamer
A gamer is, in a general sense, one who plays and enjoys games, but the term usually refers to computer games or video games.
One interpretation of the gamer psyche is a very common stereotype, usually pinned on the adolescent male: usually a high-school student, who sits staring at a computer monitor almost 24/7 with a bag of chips or a can of soda present. His only friends are those he meets on the internet, except for those he invites to have LAN parties. He is assumed to speak online in "1337" (or as it is pronounced, "leet"), a "language" made up of characters and numbers that resemble letters.
(Note: there are many different types of gamer, the above is merely the one of the more common ones.)