Someone who plays video games as a primary hobby. They tend to spend large amounts of time playing games, often in excess of two or three hours a day.
Hardcore gamers tend to care less about graphics then casual gamers. While some specialize in a single genre, they typically have fairly diverse taste in games, frequently playing a wide variety of games from different genres. They'll often seek out obscure and older games, based on word of mouth or positive critical reception. Hardcore gamers put good gameplay above all else, and don't mind if a good game has poor (or even nonexistent) graphics, sound, characters and plot.
The stereotypical hardcore gamer is out of shape, and has poor hygene. They spend a lot of their disposable income on video games, consoles, or PC upgrades, and a large percentage of their free time either playing video games or discussing them, online and off. They wear either black clothing, or T-shirts with geeky pop-culture references, often to video games. Their social skills may be sub-par, and they may be less likely than others their age to have many friends, a jobs and a girlfriend/boyfriend. Some of these stereotypes are negative, and they certainly don't apply to all hardcore gamers, but there's a fair amount of truth to them.
"John plays just about every game that comes out. He says he's a hardcore gamer, but I just call him a dork."