Castling is a special move that may allow you to safely initiate an attack within just a few moves.  It involves the King and either Rook, and is permitted only once during the game and under specific conditions (see the rules below).  In castling, the King moves two steps toward the Rook, and the Rook leaps over and next to it.  This strategically places the Rook into to the middle, from where it can attack more easily, and simultaneously moves the King to the safety side of the board.  

If castling involves the Rook closest to the King, it is known as "castling short" or castling King's side. This move is recorded as 0-0.  When it involves the Rook closest to the Queen, and hence furthest from the King, it is known as "castling long" or castling Queen's side.  This is recorded as 0-0-0.

Rules for castling:

1. King and Rook must be in their original squares and never have moved
2. Squares between the King and Rook must be empty
3. The King cannot be in check, move across check or be into check