Typically, a heckler is a person who harasses and tries to disconcert others with questions or challenges. In a public setting they are often known to shout disparaging comments at a performance or event, or to interrupt set-piece speeches, with the intent of disturbing performers and/or participants.
I first learned about hecklers when I saw comedian Don Rickles doing stand-up comedy. Don used to perform in supper clubs around upstate New York... the Catskills.
If someone from the audience became vocal, he was quick with the one-line comebacks to the point of insulting hecklers literally driving them out of the venue.
I've seen some videos where he egged them on as it was possibly a part of his routine!
In the virtual world things are a lot quieter. People blog and hecklers who are often referred to as trolls go from blog to blog (or forum thread) leaving comments to 'show themselves' with other members of the blogging community.
The psychology of it all... well, I believe something is bothering them in their personal life so they seek enjoyment attempting to disrupt the lives of others. Sometimes, it's a group and sometimes they single out a member or two.
I heard of one heckler who was bounced from a comedy club a few times so he decided to stand out front complaining to the patrons about one comedian "Don't waste your money, this guy tells the same jokes every night!"