The knight's advantages are its ability to jump over pieces, its ability to threaten pieces from where they cannot capture it, and its great potential for stealing rooks and queens. Its disadvantages are that it must be close to its target, and it can sometimes be difficult to put it in the place you would like it to be.
Keep in mind how many moves it will take a knight to get somewhere when deciding to move it, or when moving a piece out of a knight's way. The "farthest" you can move a piece from a knight is 2 diagonal squares away. It takes a knight 4 moves to get there. Second best is 1 square away (not diagonally). It takes a knight 3 moves to get to the square it is next to.
A knight can also be used to compensate for the absence of a bishop. Since a knight switches colors every time it moves, you can use it to attack or defend against either of your opponent's bishops.