Hardcore - skilled player, who accomplish a lot in game. Who is ready to sacrifice his social life, family, job for sake of progression and being best of the best, who understand value of gear and achievements and is taking game as primery job.
Casual - unskilled player, usually taking game as game and nothing else, troll, not ready to put hours to learning curve or put aside social life. Person who plays for fun and doesn't understand realy value of mmo.