In my opinion, casual gamers have a method of playing games and sometimes play them. Hardcore gamers play games A LOT. But, I also think there are a few sub-categories of hardcore gamer. The first is the freak in a darkened room, removing himself from society because games are more fun. The second, and more common, are people that often play games competitively, such as the infamous MLG gamers. They play games and are very good at them, so much so that they are the "pros" of gaming. This category also has the less impressive hardcore gamer, who simply excels at one game or type of game and plays that type exclusively. I think this counts as a hardcore gamer even if they only play games as much as casual gamers.

Most people are something like myself, I think. The middle ground. I enjoy and play a variety of games, and can pick up any game and become good at it in a short amount of time. However, I'm certainly no pro, but I don't care because that's not why I play them.

I think everyone has moments where they become a Tier 3 hardcore gamer. Sometimes, we're enjoying a game so much that we put a bizarre amount of effort into it; taking notes, formulating plans etc. I've done this with Fallout 3 and Oblivion, trying to create a god-like character*. You could say I did the same with Halo 3 for a while, trying to get all the achievements (MUST. GET. KATANA. DON'T. KNOW. WHY. BUT. MUST.).

*I totally got it in Oblivion, for the record. 100% Resist Magicka and 100+% Reflect Damage. xD