Bishops are the only chess piece to only move diagonally. Another way to consider the bishop's movement is that whatever color square it starts on, it must always stay on that color.
A bishop can be moved any number of spaces if a player chooses to move it, as long as it is moved diagonally. A bishop has a value of three pawns, according to the Capablanca & de Firmian valuation, which puts bishops at an even value with knights. This valuation is founded on the fact that the bishop can threaten between seven and 13 squares due to its movement options.