LiveJournal is an online journal hosting site. LJ's English-language content tends to be more diary-like than other blogging sites, and as such it has a (not totally unwarranted) reputation for being frequented by whiny teenagers. Its format is user-friendly and highly customizable, offering multiple user pics, mood settings, and journal layouts. Because of its more personal approach to blogging, LJ attracts a lot of teenage girls. It also attracts a lot of drama. The two are possibly related.
The site is also greatly used by fanfiction writers, possibly due to the comments section, which makes each entry something like a miniature forum. As such, LJ is home to many communities, which discuss everything from knitting to politics. Mainly, though, it's fanfiction. A lot of fanfiction. LiveJournal seems to specialize in writing communities, of which it has hundreds, most of them fanfiction. Roleplaying communities are also popular, all of them basically fanfiction.
LJ's popularity has dwindled in recent years, with the advent of first Myspace, then Facebook, and then Tumblr, which attracted a lot of the slash fangirls who previously used Livejournal. It also garnered a reputation for banning journals because of "objectionable content" which violates the Terms of Service; this is called TOS'ing.