You know these people, you may even be one of these people. Shy and retiring when face-to-face but put you in front of a key board and you don't mind chatting away or speaking your mind.
But come on, we all know this word is really an extrapolation of John Suler's online disinhibition effect http://www-usr.rider.edu/~suler/psycyber/disinhibit.html