Casual games are the games that are designed for an average gamer or computer user, they feature a very short learning curve, have very basic rules and do not require much strategy, anybody should be able to enjoy casual games within minutes of launch.
Since the first days of Windows operating system top casual games were Solitaire, Minesweeper and Tetris like games which regardless of their simplicity cost worldwide economy millions of dollars in office productivity due to their addicting nature.
Today there are several major casual game developers such as Big Fish Games, Relexive, iWin, RealArcade and few other publishers who deserve most credit for great success of the casual games. Not only do they output several new and revolutionary games daily, keeping all the casual and hardcore gamers playing all the addicting games non stop, they also do a great job as far as promotion of the games is concerned.
Casual game developers are creating very high quality games which they offer trial versions for, with either time limit, level limit or some other feature limitations of the game. Those trial versions are usually enough to have the gamer hooked and either keep playing the limited version or purchase the full version of the casual game.
Another way casual game developers promote the games, is by having those same casual games available in flash or shockwave on online gaming site and then allow gamers to download the own "offline" version of the game.
Regardless of simplicity and the basic nature of casual games, they are really taking off and becoming a larger share of gaming industry daily, who knows maybe thanks to all the new casual games we will finally let the basic Windows Solitaire go.