One of the "classically" shaped menorah with the shamash higher and in the center

The shamash or shammus is the "helper candle". It is the ninth candle for the Hanukkah menorah, also known as the Hanukiah. It is lit before the prayers are said and the other candles are lit.

When counting candles, you could almost consider it candle zero and count like some mathematicians and programmers.

Alternative spellings: shamash, shammus

an oil burning menorah with the shamash set off the line and to one side

The shamash is often 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's just become customar for some.