It's definately a subject that is hard to define.
I'd simply say being a casual gamer is when someone plays it casually and not too often, this does not necessarily mean that they aren't good at gaming (as some people like to use the term for), just that they tend to play when they feel like it!
I have a few friends that play games a lot, loads in fact! They're hardcore gamers, always at the control pad... but I wouldn't say they were natural gamers (if there is such a term).
Simply for the fact that usually when I play them, I beat them! I can spend half the time they have spent on a game and be better than them at it, even though they tend to play more than me... they even say so themselves!
Casual gaming is not what some people make it out to be, I find that I tend to game a fair bit, more than what most people would call casual, maybe even hardcore.
Great topic!