The small group of individuals that frequently and strongly voice their opinions, constrasting with the silent majority that makes up most of the population base, but rarely give their opinions.
Mostly used to refer to forums of a particular service or product, such as a video game.
Vocal Forum Poster : Oh my God, I totally hate all the changes that have been made to this game! You all suck!
Game Developer : Noted. But we're not going to undo the changes, because only the people that don't like them would take the time to come complain, the vocal minority of players. Most people are happy with the changes, and wouldn't stop playing the game to come post on these forums.