The term "gamer" itself is harmless, but Generally if you are labeled "Gamer" by a non-gamer, it's usually bad.
How we use it, One whom enjoys Video Games, preferably over other forms of media.
How they use it, One whom does nothing but play video games (this usually includes a social out-casting of some sort)
Its all a big hypocrisy really, I think in general, Gamers are trying to close the social gap by using buzz-words that sound more pleasant.
For Example, The Jock who plays Fantasy football picks on the D&D player, calling them a nerd... even though Fantasy Football is basically a re-skinned D&D to a sports theme.