Lets see if I can add my spin to it. I won't reference to any specific games.
Casual - plays games that involve very little learning curve and can be enjoyed immediately and with small gaming sessions. Also describes those who choose games based on what "everyone else" is playing, mostly FPS because violence is cathartic.
old school gamer - someone who has played games steadily since before the PS2/GCN/XBOX era (this is my opinion).
core - these people tend to be more informed about what is going on in the world. They tend to vary their gaming experience to other than what the majority of gamers play. They tend to prefer games that offer a decent challenge.
hardcore - these gamers are probably also old school gamers. They look for new and innovative games who can offer them a fresh experience. They prefer games with a strong challenge (preferable variable challenge). They are very well informed as to what is happening in the gaming world, and often plays niche titles.
That is my take on it. Playing a game online until you are rediculously good at a game does not make you hardcore. Being able to pick up a game and be good at it in a short time does.
