There are currently five of them.
News Announcements (lj-news)
This is a announcement only list to inform the LiveJournal community of new features, events, and other general information.
General Discussion (lj-general)
This is a discussion list regarding the use of the LiveJournal system. Users can post questions, hints, tips and most likely general conversation.
Style Discussion (lj-styles)
This list is for the discussion of the LiveJournal style system including how to create styles, HTML assistance, and style debugging.
Development Discussion (lj-devel)
This list if for the discussion of LiveJournal development. Topics can range from "Which is better Perl or PHP" to "MySQL tuning parameters". Mainly for the technically minded portion of the LiveJournal community to assist in future development of the LiveJournal system.
Business Dicussion (lj-biz)
This is an approved subscription list for discussion of LiveJournal business topics. Topics can range from "advertising revenues" to "the number of paid accounts versus free accounts".
The mailing lists are publicly archived for your viewing pleasure. Subscription instructions, archives, member lists and other useful things can be found at http://lists.livejournal.com/.