A listserve is an email list that you can join (subscribe) or leave (unsubscribe). Once you are a member, you can post messages by sending an email to the list. All the members of the list receive your message and can reply. Replies go back to the list (be careful here if you are sending a private message, make sure to address it to the person not the list)
Many lists (including the NWOCA ones) have a web archive where you can view and search through past postings.
Lists comprise lots of other people who share an interest. NWOCA has lists for Apple, Windows, FileMaker Pro, Smartboard and Moodle users.
I am also on a couple of other lists that are useful for keeping up with Educational Technology including the EDTECH list and the e-TechOhio Tech coordinators list to name a few.