Mainstream - Games here are pick up and play, they tend to encompass the largest potential audience and can be quite easy to play but may still have depth for those who wish to master them.
Examples : Mario Kart, Kameo, Super Mario Galaxy, Wii Sports, Wii fit, Guitar Hero/Rock band
Core - Assumes and requires some prior knowledge from the gamer but can be picked up by anyone willing to give a little time to learn whats required of them. This genre encompasses a lot of your bread and butter franchises, especially from the HD consoles. You can put most shooters/RPG games/Simulation racing games etc into this category.
Examples : Halo 3, Call of Duty 4, Team Fortress, Portal, Warcraft, Zelda, Metroid Prime, The Sims
Hardcore - Requires specific knowledge of the game or genre for people to play. Games like Fire Emblem, Metal Gear Solid 4 and Ninja Gaiden 2 are examples. Can be very difficult, especially for those people new to the genre/game.
These aren't hard categories, its more a scale where 1-3 might be mainstream 4-8 Core and 9,10 for instance and it doesn't include my most hated word - Casual!
The categorizations I have made here are from the perspective of people wishing to pick up the game and play them, who might be willing and able to. People who are playing games are always going to pick up the subtle cues which let them play more difficult games, if they want to in the future. For example - Someone who has played and finished Heavenly sword would be much better prepared to play Ninja Gaiden than if he/she hadn't.
Tease.
