The knight chess piece can be a powerful piece because it has the ability to "hop" over other pieces. You get two of them at the beginning but don't waste them.
The knight chess piece can move one space horizontally and two spaces vertically or two spaces horizontally and one space vertically. This piece can access every space on the board but it is much more complicated to do so then the rook or the queen. It can take up to 4 turns for the knight to reach a specific space on a board.
Using the knight can take quite a bit of planning. Often times it may be necessary to memorize the number of moves it take to get to a specific position. Of course the big advantage to moving the knight lies in the fact that it can not be obstructed by other pieces. The knight is found in many end game scenarios but probably less frequently so the rook or bishop.
Chess pieces can come in many styles so the knight will look different from set to set. Staunton style is the one most people are probably familiar with since this is the tournament standard. The knight in Staunton style set look like a chess horse.
When looking for chess sets often times it's worth looking at how much detail is in the knight. Chess pieces are often hand carved and in styles such as the Staunton style the knight will normally have the most detail. However, in other styles, especially in theme sets, pieces may have it's own style. The more detailed the pieces the more value they hold.