Hard core honestly is a mind set. It is when someone consider it to be more then a game. They look at something and find out how to maximize the outcome. They put in huge amount of time. Some aspects of real life no longer take priority over the game. The game is no longer about just being fun. They want thier hard work to be rewarded over those who do not put in the work.

Casuals: They treat mmos like any offline game. They just want to have fun and socialize in a multiple setting. Life > Game in almost all aspects. The do not want to invest a lot of time into the game to get the most out of it.