It is a small, ugly creature who typically lives under bridges.
No, seriously, it is a person who comes and posts for the sole reason of stiring up trouble. For example, if someone came and posted to the gentle discipline forum that they spank their child and see nothing wrong with it, there is a good chance that they are trolling (or really haven't checked out our site to see what we are all about.) Trolls like to post things they know will be inflamitory and watch the result.
The best thing to do if you suspect someone is a troll is to ignore them and contact a moderator.