How do guilds work?
They were something like trade unions, only much more powerful. A carpenter would be part of the carpenter's guild, and nothing would get built if they weren't get involved. They weren't really owned by anyone, they functioned like a collective and handled all the transactions that came their way. If someone wanted a table built for example, they wouldn't shop around, they would simply go down to the local carpenter's guild hall and arrange for it's construction. They held a monopoly for their respective trades and no one could do business outside of them. And while the guild members weren't employees as such, they weren't independent businessmen either.
PS: Here's a Wiki link where they're described in more detail:
http://en.wikipedia.org/wiki/Guild