The "core vs casual" differentiation is somewhat of a marketing concept that illustrates the "80-20" economic rule. You make 80% of your business with 20% of your clients and vice versa. Because, let's face it, the core users group is also where a good chunk of the profit is, provided you have a good strategy to convert their passion into dollars.
In theory, the core user of an iPhone is who will spend the most in service fees. The core user of a pizza delivery service is who will order the most pizze. The core user of a porn web site is the compulsive masturbator who will subscribe for the whole year. The core user of a drug is an addict. The core user of a game console is who will buy the most games and peripherals [if he is not a big time pirate]. He is who pushes the attach rate up.
In the gaming jargon though, the term "hardcore gamer" has evolved in the early nineties when adopted by the console community to refer to their co-opted elite. It's a blurry word that would somehow apply to a superior breed of gamers who supposedly would know more about games because they spend more time on them.