As Nikkom and Onyx have shown (and as I have said before too) there are no casual game, only casual players.
In truth I don't think it is quite as simple as that, There are games that are good for casual players, Like Mario Kart and Wii Sports etc... but those are not "casual games" because they can be played in a much different way by us avid gamers.
However there are some games I think, that because they don't really contain much, cannot really be played any other way than casually. Some examples of this would be quiz games like Buzz, as well as the more substandard of minigame compilations like Game Party and Sports Party (Deca Sports in USA) I wouldn't even put Carnival games into this as from what I have seen it has a little more substance.
Personally I would break games down into 4 categories
1.Hardcore - games that are fiendishly difficult and complicated from the get go, cannot be played casually, Fire Emblem is the only example I can think of.
2.Core - Most games are in here, Most shooters and adventure games, GTA, Mario Galaxy, Banjo, Ratchet, Uncharted.... Can be difficult for Casual gamers to get a grip, but are accessible enough for most players and usually let people in gradually.
3.Bridge - Many games that are considered "casual" would be in here, they are designed to be accessible to all, but have enough depth to keep better players going, Wii Sports, Mario Kart, Guitar Hero, Wii Fit
4. Casual - Not many games are purely casual, these games can be fun for anyone, but should be played in short bursts and don't have much to add to what you play in the first couple of hours. The Eyetoy games, Buzz, Wii Play, and some of the poorer quality party games.
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Of course some games can be a mix of two of the above, and things like Wii Fit and Brain Training can also stretch out to a fifth category that isn't really it's own, called "non-game" (there are no "non-games" alone, but some games in the 3rd and 4th category can stretch out to it, Wii Music from what I can gather will be a category 3, but like Wii Fit will be part of "non-game" too (though less so than Wii Fit and Brain Training)