The king is the most valuable/important piece. The object of the game is eliminate your opponents king by trapping it so that its escape is not possible, this is called checkmate. Check is when your opponents king is threatened with capture but your opponent can safely remove his king from danger on the next move. If your opponent has no safe move from check, the king is said to be in checkmate. The King it is usually the weakest piece, until the endgame.
The King and Rook may make a special move called castling. The king moves two squares toward one of its rooks and then the rook is placed on the other side of the king. Castling is only allowed when:
-neither the king nor the castling rook has been moved
-no squares between them are occupied
-the king is not in check
-the king will not move across or end its movement on a square under enemy control