Lolicon, (or Lolikon/Rorikon), is basically the japanese word for a pedophile who is interested in young girls. It is basically a japanization of "Lolita Complex." It is lolicon instead of lolicom due to the absense of the "m" sound, and the presense of an independant "n" sound.
Overseas, the term can also refer to the type of anime, manga, or doujinshi which involves young girls under the age of consent involved in sexual activities, though usually 12 and under. Such pornography is normally refered to as loli in Japan.