Final Fantasy XI, fleets in Star Trek Online, kinships in Lord of the Rings Online, but the game could call them bagel and tea parties if it wanted to -- for most players, it's a guild, and no matter what, the core idea is the same. Only a handful of games don't have tools and support for player organizations, pooling resources and skills together so that everyone can get further than they could alone. At the best of times, a guild is a network of people you can call on at almost any time to help you with your goals, and the same holds true for your fellow members.
At the worst of times, however, a guild is a mechanism for bringing more drama into your life than dating a paranoid schizophrenic. World of Warcraft's new Dungeon Finder has even prompted some questions in the game about how necessary guilds even are at this point. So how important is your guild to you? Are they your stalwart companions or just some folks? Would you leave the game without them, or do you think you'd probably end up playing more?