Lighting the candles is the main ritual of Hanukkah {חנוכה}. Because Hanukkah lasts eight days, there are eight candles on the Hanukkah Menorah (“hanukkiah” {חנוכיה} in Hebrew), and there is one more candle– the “shamash” {שמש} (Hebrew for attendant) which is used to light the other candles.
The number of candles changes according to the number of days – which means, at the first night one lights one candle, on the second night two candles are lit, on the third night three candles are lit and so forth.
The original Menorah, the one from the Temple (which can be seen on the official emblem of the state of Israel) had ‘only’ seven candles.
Because Hanukkah has eight days they added another candle to the Hanukkah Menorah!
Happy Hanukkah!