The ninth candle on the Chanukah menorah is called the Shamash. Using it to light the other candles is a continuation of the practice followed when the seven-branched candelabrum of the Tabernacle and Temple was lit.  The seventh branch in each of these menorot was called the Shamash, meaning "servant."  Used to light the others, it was not counted as one of the lights. 

A ninth candle is also used because the eight primary candles of the Chanukah menorah may not be used for practical purposes.  By having a ninth candle to light the others, one will not be tempted to use any of the eight primary candles for such purposes.