School me on guilds....

[Swordsman]

I've been doing about 90% solo so far, and have made it to Notoriety 24 over the last month or so (and recently finished the Scary Mary quest). I've been invited to join guilds a couple of times, always by a random person that I've never seen before. I've declined, because I don't want to join up with anyone that I don't know. Also, I don't really know the reason to join a guild. I'm mainly playing the game for fun and adventuring, not really to chat a bunch and meet new people. I don't mind doing that, it's just not a big motivation for me.

So what are the pros and cons of guilds? Why would I want to join? What are the benefits? What do guilds do? How would I go about choosing which guild to consider joining? What are the differences between guilds? Why would I want to join an existing guild rather than creating my own (or vice-versa)?

Thanks in advance for the help!