Castling is one of several special chess moves.
It is the only move that allows two pieces to be moved in a single turn.
Castling involves both the king and one of the rooks and involves moving the king two squares toward the rook and placing the rook in the square that the king moved over.
In effect, the rook "jumps" over the king.
There are a number of rules that must be followed before a castle move can be played.
They are:

• Both the king and the rook must not have previously moved.
  
• There must be no pieces between the king and rook.
  
• The king must not be in check, move through check or finish up in check.
  

Castling can be played if the king was previously in check (but is no longer in check) or the rook is under attack, or finishes under attack.
It is also possible for the castled rook to check or checkmate the opposing king after a castle move is played.
Kingside castling involves castling using the rook that is closest to the king (the rook on the right if seated behind your pieces).
This is considered the safest move as it places the king closer to the edge of the board after the move is played.
Queenside castling requires more pieces to be moved between the rook and the king (including the queen) and it therefore not as common as kingside castling.
Castling is often played as a defensive move to place the king near to the side of the board where it can be more easily protected.
It is also played to move the rook into the center of the board where it is more powerful.
Many chess games often begin with a castle as early as possible, and it is therefore common for both players to kingside castle as soon as possible.