The rook, shaped like a castle, is one of the more powerful pieces on the board. The rooks, grouped with the queen, are often thought of as the "major pieces". A rook may be thought of as being worth a bishop (or a knight) plus two pawns. However, it will be seldom that such an obvious trade will be practical.
The rook can move any number of squares in a straight line along any column or row. They CANNOT move diagonally for any reason. In the example shown in the diagram on the upper-right, the rook can move or capture in any square that has a blue dot. The simplicity of the rook's movement is indeed what makes it powerful. It can cover a significant area of the board and there are no areas which an opponent's piece - moving one square at a time - can slip through. Note that at all times the rook has the potential to attack 14 squares. This is over 20 percent of the board. Only the queen can cover more space.
Because of the rook's position on the edge of the battlefield it usually is not used until the later phases of the game. The rook may also make a move in conjunction with the king. This maneuver will be explained in the section called castling.