Not this thread again...

The simplest way is to look at it as a matter of time.

Hardcore players devote a lot of time to the game, and use this extra playtime to their advantage (more class levels, more gil, more endgame gear, more achievements, etc.).

Casual players don't devote a lot of time to the game (because they can't, or don't feel compelled to). Casual players are content with a handful of achievements obtained within their limited playtime.