Questions relating to castling
Castling is a move in chess which can only be performed once, or else never, by each player in each game. Castling consists of one move in which a player moves his King two squares toward either Rook (henceforth termed the chosen Rook), and at the same time moves the Rook around the King to the square immediately on the other side of the King. The conditions under which castling is legal are as follows:
the King and the chosen Rook must not have moved since the beginning of the game,
there must be no occupied squares between the King and the chosen Rook,
the King may not be in check (see check, may not move through check during castling, and, of course, may not castle into check.
The symbols for castling in most notations (see notation) are
0-0 for Kingside castling (i.e. castling with the Rook closer to the King), and
0-0-0 for Queenside castling.
For more information on castling, see the Wikipedia article.