Hardcore gamer - One who truly appreciates a game for what it is, and participates in gaming as an active hobby, rather than playing games simply as a means to pass the time. A hardcore gamer tends to be quite knowledgeable of the current state and history of the gaming realm.
Hardcore game - An excellent yet under-appreciated title that gains critical acclaim in the minds of hardcore gamers, but does not manage to achieve mass acceptance from the gaming market as a whole, most likely due to a lack of advertising, the "cool" factor, or some other such nonsense.