A casual game is a game that is suited to be played only now and then and in rather short sessions. Whenever you want to have a little fun time, you can start a casual game, you're right in the middle of the action, you have fun for a few minutes and afterwards it's no problem to not play the game for weeks because a single gaming session is usually self-contained.
This leads to casual games usually having the following characteristics:
- the rules and mechanics of the game are very easy to understand, though eventually hard to master. You do not need half an hour of tutorial levels to understand even the basic actions etc., the game is instantly accessible
- a single round of the game usually only takes a few minutes and is self-contained - unlike for example a game like Zelda, where you make slow progess, the games takes dozens of hours, you do not play through in a single session. This also leads to casual games not having much of a story and not having a clear end.
I consider Bejeweled a perfect example of a casual game. The gameplay is understood within five seconds, and whenever you need to waste a few minutes (for example because you're waiting for the bus) you can enjoy a full game of it within that time.
So I'd say all minigames (and minigame collections) are casual games, but it's not necessarily the other way around. For example I'd consider Singstar and Guitar Hero casual games, but not necessarily minigames.
