"But just what is a geek?" you ask. Well, I'll tell you. At least, I'll tell you my definition... which may be different from "general usage" of the term, webster's definition, and even the definition used by other geeks. (note: over time, Webster's definition was eventually updated to include elements of this new interpretation)
A geek is someone who spends time being "social" on a computer. This could mean chatting on irc or icb, playing multi-user games, posting to alt.sex.bondage.particle.physics, or even writing shareware. Someone who just uses their computer for work, but doesn't spend their free time "on line" is not a geek. Most geeks are technically adept and have a great love of computers, but not all geeks are programming wizards. Some just know enough unix to read mail and telnet out to their favorite MUD.
Geeks are generally social outcasts from mainstream america. The ranks of geekdom are swelled with gamers, ravers, science fictions 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. Geeks prefer to socialize with other geeks, the self proclaimed weird. Therefore they go online to organize parties, food runs, drink runs, and movie nights, and be assured that their companions would rather talk about superheros as modern mythology than the latest football scores.
Geeks are their own society: a literate, hyperinformed underground. The community accepts people from all walks of life, assuming they have access to the net and the skill to use it. Geeks are rather openminded with regards to nonstandard lifestyles. Many geeks are queer, more practice non-monogamy, and the most common religion is neo-paganism. You can't tell if someone is a geek just by looking at them, there is no dress code. Some dress casual, some prefer silk - but few pay attention to current fashion. You are more likey to see a geek in a renaissance bodice than a dress from glamour magazine; or a tiedye instead of suit and tie.
The unwritten geek credo states that originality and strangeness are good, and that blind conformity and stupidity are unforgivable.
Take care not to confuse the terms geek and nerd. A nerd is a person with no social skills, usually obsessed with science or technology (geek is more computer specific). Nerds are known for their pocket protectors, taped glasses, and plaid shirts. Many nerds are also geeks, using the net as a safe screen to hide behind while practicing their social skills. However they rarely come out to be seen in person at live geek events, so there is little reason to be concerned.
The term hacker tends to refer to the more programming intense set of the geek crowd. However the term is overused in the popular media, and therefore is no longer much used among "real geeks". Hacker also has negative connotations related to cracking, or illegally obtaining access to computers and accounts.
Geek can also be used as a verb. "To geek" is to sit online and read mail, news, chat, and otherwise waste time in front of a keyboard. This "geeking" often consumes many hours, even if the intention was to "just log in and check my mail." Some would say this time would be better spent being social in person or even just being curled up in a sunbeam.
There was once a special breed of geeks known as b-geeks after the computer ucscb.ucsc.edu, where they gathered. However, many of these b-geeks graduated, more failed out, and ucscb has waned in importance in the ucsc computer system. Now spread to the winds of computer access, they still gather in electronic forums and do lots of fun stuff. The group is now known as scruz-geeks, and you can find out more about them on the santa cruz geek social scene page.