In chess, a rook is a type of piece that can move any number of squares horizontally or vertically. In terms of relative value, rooks are the second most powerful pieces after the queen, and are considered to be worth approximately five pawns. Rooks are not very valuable in the opening, because they start out blocked by other pieces, but their value increases in the middlegame and then they become extremely powerful in the endgame, once many ranks and files have opened up by the capture of other pieces.
The English word "rook" comes from Persian rukh meaning "chariot." In western standard chess, rooks are almost universally represented by a crenelated tower such as that found in medieval castles (which sometimes leads rooks to erroneously be called "castles" by the uninitiated). However, in older forms of chess played in India, Persia, and other parts of Asia, rooks were instead represented by wheeled chariots. One theory is that the change occurred as the result of a false etymology by comparison with Italian rocca, meaning "tower," when chess was first imported to the West during the Middle Ages.