The shamash or shammus is the "helper candle". It is the ninth candle for the Hanukkah menorah, also known as the Hanukkiah/Chanukiah. It is lit before the prayers are said and the other candles are lit. Shemash was the ancient Babylonian sun-god.
When counting candles, one should consider it candle zero and count like some mathematicians and programmers.
The shamash is always set off from the other candles. Usually it is set off by spacing - height, a gap between it and the other candles or out of the line created by the other candles. The color of the candle used for the shamash may also be different from the other candles. This is not required; it has just become a custom for some.