From a developers standpoint a casual gamer is someone who does not have the time/motivation to play for longer periods of time.
Let me explain this a little further. A casual gamer would be someone who logs in for a hour or two a night and plays. This is not always the case but again I am telling you this from a developers point of view.
Another person who we would call "casual" would be someone who really doesn't want to pvp and doesn't want to raid, primarily due to the amount of time required for these activities. This leaves dungeons, which this type of player may do occasionally especially if the dungeon is shorter and consumes less time.
There are other characteristics but, you kind of get the idea. This is again just a view of how some of the companies I have worked at look at a "casual" gamer.