A person who has chosen concentration rather than conformity; one who pursues skill (especially technical skill) and imagination, not mainstream social acceptance. Geeks usually have a strong case of neophilia. Most geeks are adept with computers and treat hacker as a term of respect, but not all are hackers themselves - and some who are in fact hackers normally call themselves geeks anyway, because they (quite properly) regard `hacker' as a label that should be bestowed by others rather than self-assumed.
One description accurately if a little breathlessly enumerates "gamers, ravers, science fiction fans, punks, perverts, programmers, nerds, subgenii, and trekkies. These are people who did not go to their high school proms, and many would be offended by the suggestion that they should have even wanted to."
Originally, a `geek' was a carnival performer who bit the heads off chickens. Before about 1990 usage of this term was rather negative. Earlier versions of this lexicon defined a `computer geek' as one who eats (computer) bugs for a living - an asocial, malodorous, pasty-faced monomaniac with all the personality of a cheese grater. This is often still the way geeks are regarded by non-geeks, but as the mainstream culture becomes more dependent on technology and technical skill mainstream attitudes have tended to shift towards grudging respect. Correspondingly, there are now `geek pride' festivals (the implied reference to `gay pride' is not accidental).
See also propeller head, clustergeeking, geek out, wannabee, terminal junkie, spod, weenie, geek code.