Hanukkah, meaning "Dedication", is a Jewish holiday that occurs sometime in November or December (it starts on 25th of Kislev in Hebrew calender). In 2005 it falls on December 26. It is celebrated over a period of eight days in commemoration of the Jews' victory over the Hellenist Syrians in 165 B.C.E.
In 180 B.C.E. Antiochus came into power as the Syrian King and gradually made Jews follow Greek traditions and worship Greek gods. After years of struggle the Jews, led by Judah the Maccabee, beat the Syrians. The story goes that upon re-entering the holy Temple in Jerusalem one Menorah (candelbra) was lit. Although there was enough oil inside to burn for one day it miraculously lasted eight days. This is why Hanukkah is celebrated over eight days by lighting candles every night of the holiday.
It is not one of the more important holidays for the Jewish, but its closeness to Christmas has commercialized it somewhat, especially in The United States.