What is Castling?

Castling is a special move involving the king and a rook of the same colour, the only move in chess that moves two pieces simultaneously. The king moves two squares toward the rook, and the rook moves to the square the king crossed. Kingside castling (O-O) moves the king to the g-file; queenside castling (O-O-O) moves it to the c-file. Castling is only permitted when neither piece has previously moved, no pieces stand between them, the king is not in check, and the king does not cross or land on a square attacked by an enemy piece.