In the most all encompassing definition of Hardcore vs. Casual, I would say this is the line: Hardcore gamers actively care about video games, casual gamers may play video games, but they don't care about them. Casuals don't nitpick, or compare, or know who the developers are, they play a game and either think it is fun or don't. Casuals don't know about what games are coming out (as someone mentioned) or what new system is about to be unveiled. They don't care. Under this definition, the term "Casual Gamer" is a misnomer, because you'd be hesitant to call them a "gamer" at all. (I personally don't like the term Gamer, but that's another conversation)
I think the gap between these two types is the most distinct, a person can go from being casual to hardcore in a matter of seconds, but once the gap is crossed the change in attitude towards video games is very distinct. As we get into categorizing within the aforementioned hardcore category, the distinctions get less defined. Examples being quibbling about who is more hardcore: who has more games, who is a bigger fan, who plays the game for 8 hours a day, etc., etc.