I like Gihja's answer, so I'm going to expand on it a little.
A guild is the union of a group of players that support each other through such game-related issues as leveling, questing, raiding, tradeskilling, outfitting each other, giving advice, and generally being a set group of people to talk to. They are, in essense, your in-game friends that you work together with, and the amount to which a particular guild favors each of the items listed above varies from guild to guild.
You should consider the demeanor of the guild before joining it. Being in some guilds will help your reputation as a player. Some guilds are very well-regarded and others are ill-reputed. People's second impression of you, after reading your name (which hopefully isn't a retarded internet-type name,) is your guild. You should also be aware that some people take being in a guild very seriously, and leaving it as well.
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Only the exceptions can be exceptional.