A person who only uses the most mainstream of items related to the thing they are using. Often casuals will annoy more hardcore fans of things
Billy: I just got a new computer with X amount of ram and a Y brand processor!
Sean: Woah dude, I don't know what that means I'm only a casual!
Billy: *Glare* "He told me was good at these things, but in the end he's only a casual user..."