Bishops are generally accepted to be the third most valuable piece. Their advantage is that they can get around the board quickly, and can get through holes in enemy lines. Although bishops and knights are both valued at 3 points, many players consider bishops to be just slightly more valuable than knights.
Keep in mind, however, that bishops often tend to be the better piece to have in the end game, because a knight can take several turns to get to where you may need it to be. The bishop's biggest advantage over the knight is its ability to act in long range operations. Its biggest disadvantage is that it can only attack or defend on half of the board's squares. It can be quite frustrating when you only have a light-square bishop and all of your opponent's pieces are on dark squares.