Hardcore Gamer: Gamers that like the same things that I like.
Casual Gamer: Gamers that like things that are stupid and wrong.
That's very funny.
OtoH, To take the subject more seriously, I think it's more:
Hardcore Gamer: Has played for decades and/or very intensely (IE spends a lot of waking time prepping for and/or thinking about the game. May or may not post on boards about it, etc. Has probably spend a lot of money on the game. Certainly has spent a lot of time on it.
Casual Gamer: Even
if they've played for decades, doesn't spend a lot of time outside the game thinking about it. May or may not know much about any aspect of the game that they don't directly play with, may or may not have spent a lot of time or money on it; probably does NOT post on message boards about it, and if so, not often.
The difference can be subtle, and some people can be hardcore at times in their lives and casual at others.
To use my personal IRL group, I'd say that I'm the only Hardcore member, but my current group ha played together for 14 years, and a couple of them have bought every 4e and 5e books. One of them (the
most casual, in that he only makes less than half the games and isn't super familiar with the rules) spends the big bucks on the expensive "miniatures" (like the mounted dragon heads and the big Terrasque) to fill his "man-cave", but isn't all that versed in the
game itself, while another one spends almost
no money at all but has an encyclopedic knowledge of Realms lore. (And yet I would
still count him as a casual gamer - he wouldn't be caught dead on here, for example).
I know their spending habits because, of course, as a retailer, we play after hours at my comic and game store, and they buy all their stuff from me!