While it is probably the best known Jewish holiday, Hanukkah is not the most important Jewish holiday. None-the-less it is still an important one.
Hanukkah is known as the festival of lights and is a celebration that stems from a group of Jewish fighters, known as the Maccabees. After their temple was defiled by the Hellenistic Syrians, the Maccabees were able to eventually "win" the temple back. When they went to find oil to light the temple candles, they only found enough for 1 night. Miraculously, the candles all burned for eight nights from that one little jar. The following year, the rabbis designated those days (starting on the 25th of Kislev (November/December)) as a holiday to celebrated with praise and thanksgiving.
This is a brief history, but you can find more at the following websites: