The Hebrew letters on each side of the dreidel, נ (Nun), ג (Gimmel), ה (Hay) and ש (Shin), stand for the Hebrew phrase "Nes Gadol Haya Sham" or in English "A great miracle happened there." The miracle being that of the Hanukkah oil in Jerusalem, which lasted for eight days instead of one.
Some say that the dreidel game has its origins during the period when the ancient greeks outlawed the study of the Torah. When Jews were not free to openly practice their religion, they gathered to study the Torah in secret but would bring a top with them. If soldiers appeared, they would quickly hide what they were studying and pretend to be playing a simple game.