The bishop has a special characteristic. It always remains on squares of one single colour. In chess this results in many strategic consequences. It can develop some power along free diagonals but it can only ever reach half of the squares on the chessboard.
In open positions two bishops are very effective because together they can control a lot of squares. Tow bishops cooperating like this are known as the bishop pair. Because the bishop pair can work so well, in practice the bishop is a tad more valuable than the knight.