Knights are very special because they are the only pieces on the board that can jump over other pieces.
Their move has the shape of an L.
Knights should be developed immediately in the opening because of their ability to jump over other pieces.
A knight in the corner has only two possible moves while a knight in the center has 8 possible moves. This makes a knight in the center much more mobile and powerful.
A knight that is far from the opponents pieces offers no threat at all because it's not a long range piece. The is not effective to move to the other site of the board quickly.
Knights are still very effective when they are placed near the action.
Knights are very good pieces to use for a fork in several directions. A fork is a situation when one piece can attack two or more opposing pieces at the same time.
This way, you can force the opponent to save one of his pieces and sacrifice the others.
It's very hard to predict the movements of a knights. Especially inexperienced players have problems with it and they forget to watch out for a knight.