This just in, fresh off the interpress - A casual is anyone who:
-Enjoys games
-Uses a console (plebs)
-Doesn't play competitively to win
-Likes things I don't like
-Lives above sea level
Spoiler Being entirely facetious, as I believe Igino is.
That said, a good deal of the fanbase and likely the forum is going to be casual or low mid-core gamers that have gotten into slightly more mid-core games because of Skyrim, simply because it was made to appeal to a wider fanbase and what that means at this point with the proliferation of the silly little phone games everybody and their great grandmother are playing is appealing to casual gamers.
On a serious note, I play on XBox because I cannot afford a good PC at the moment and my old one bit the dust. I will say this: The only way to play an Elder Scrolls game to its full potential is on a PC, due to the sheer number of bugs if nothing else, and secondarily because of the mods (which are, admittedly, about 1/3 of the draw of the games for me if not more).