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.

It is much simpler than that ... The "Casual" vs. "Hard-Core" debate goes back pretty far and the first time I encountered it was in reference to Massively Multiplayer Online Role Playing Games.

Most of the people who played these types of games did not devote the insane hours to the games that a small demographic did, but (with how these games were designed) the only way people are rewarded is based on time spent. People wanted to get a fair (not necessarily equal) experience from these games being that they were paying as much to play the game as everyone else was; they figured that a person who played these games casually should still have an opportunity to take part in all of the content. Of course, those who dedicated 8 to 12 hours per day on these games did not see this as being (in any way) fair and started deriding anything that had less investment into it by calling it “Casual” (with many of them bashing WoW for being the most casual MMORPG ever).

In DaoC, Hibernia was the newest and most broken realm upon release and had (by far) the most broken classes; at the same time, it was designed with the most colourful and “cartoonie” (for lack of a better word) art style. Because of its issues there were (constant) forum posts about the issues and those who were “protecting” their realm argued that the colourful art style of Hibernia attracted the “Casuals” and their whining was not representative of the problem.

For years this argument has been growing and mutating without any proper definitions and the result is people who think that they can (after the fact) define and categorize games meaningfully into the terms “Casual” and “Hard-Core”.

The truth is these terms are 100% BS

Squilliam Fancyson