Casual games refer to the length and time investment required for playing a game. Casual games can come in many forms but are most commonly puzzle and simulation games. Casual games are noted for their simplicity and and usually focus on just one thing.
Casual games are designed around having addicting play that has low play time. Casual games will often have no real end to them. They will require you to do just one more thing until you unlock a newer harder competition within the game. Casual games are not particularly hard to play and are accessible to anyone.
A common scheme to make casual games addicting is by posting your ranking or high score to all players. This gives the game a competitive edge so that people will know who and what they have to beat.
The casual game was first invented as the arcade machine. This casual game would have kids throw their hard earned/pilfered quarters into a box. These games offered a continue feature to hook a player into keeping up their high score but for all intensive purposes when you lost you lost.
The modern casual game is played on a cell phone or mobile device.
Casual games as a final feature often have no continuity. There may be levels but they are not necessarily attached to each other. The focus of a casual game is on a single game with no connection or continuity.