IMO
Hardcore: Someone who makes a hobby out of gaming; plays many diverse games, all different game play and story genres; really considers the elements of a game and how they come together (game play, graphics, music, story, so on); would classify video games as an art form.
Casual: plays games on a regular basis but just to kill boredom like jhazicalization said. May only play one type of game or just one game (the exception being pro gamers, I'd say their pretty hardcore). People who only play COD, no matter how much they play it, fall into the "casual gamer" category.
I guess to put it more simply, a casual game just plays games. A hardcore gamer is a bit more analytical about it. It can be difficult to really distinguish between the two.
The main point that I really want to make is that picking up Angry Birds every now and again does not make you a gamer at all, neither casual nor hardcore. It's just like reading a single book doesn't make you a bookworm and watching the newest blockbuster movie doesn't make a film fanatic.