Where does/can xbmc pull media from?

Instead of throwing more money at an external harddrive locally connected to ATV, I would like to use my 1TB external currently connected to my airport extreme base station. I guess my question is really the subject, where does/can xbmc pull content from?

Any share on your network should do, I usually have smb shares with read-ony and no user/pass, this is easier to set up.