King Vamp if you want to be as generalistic as possible, then here is an easy way how people (or the industry) label or identify gamers/themselves.
Again this is generalizing things before someone goes off on "well I'm a core gamer and I play that too..." so please understand this statement before going on about trial labels.
Casual plays Angry Birds, Just Dance, Petz, other shovelware and dabbles in things like CoD, Halo as gateway games.
Hardcore plays FPS, (Halo, CoD), Uncharted, GoW (Gears/God of War), and other more niche things.
However the line is beginning to blur if you believe any of those game tracking firms as one did a "study" here lately that more hardcore gamers are playing more casual fare (Angry Birds, Facebook games)
Frankly if you want to identify them look at their game collections, how much they play, etc. that is the MAIN difference between casuals and hardcore gamers. And YES it is a label construed by game companies to lure more consumers into the market to play. And Nintendo was the first one to target this market hard with both the Wii and DS (see early Wii and DS commercials with Mom, family, elders playing games, Ie. the segment of folks you RARELY see depicted playing any of these titles)
And there is a broad, semi-defined answer to you, whether you believe it to be true is up to you (since you seem to believe there is no such thing, which is not the case) But there is a definite divide between casual and hardcore gamers, and the casuals sometimes become core gamers or they don't like any other hobby it depends on how much time and money you put into it, simple enough I'd think.