I think I was in a buddy's guild long ago, for diablo, but I was removed and don't remember the name of it.
Is a guild exclusive to friends or what?

You can view the guild list here: http://forums.d2jsp.org/guild.php
Guild FAQ: http://forums.d2jsp.org/info.php?p=5
There are many benefits in becoming a member of a guild. (such as different contests/sharing the same interests/meeting new people etc.)

different guilds have different rules. some are for friends, some are private and require an application process to get in, and some let just about anyone in.

usually you can look at a guild's charter to find out more about the guild, and try to get to know them to see if you'd be a good fit in the guild.

Guild don't have a 'purpose' they are just a group of friends.

They can host events together, chat all the time, trade together, and are usually are trustworthy between each other.