A menorah (sometimes menora) is a specialized candle holder, or candelabra, used by the Jewish people. Two main varieties exist: one with six or seven branches and one with nine branches. The nine-branched variety is used only in relation to the Hanukkah (or Chanukah) celebration and is often referred to as a hanukiah (or chanukkiah). The design most often consists of a central stalk with a round base, with either three branches curving up on each side, and sometimes one branch extending straight up from the base.
The menorah is said to represent the burning bush that Moses saw, as related in the Hebrew Bible (or Tanakh). Historically, one was used for ritualistic purposes in Jerusalem during the time of the Temple. It burned olive oil, rather than candles. The Temple Menorah remains on record until sometime during the sixth century, after it was brought to Constantinople. After that time it vanishes, and it is unknown whether it still exists or was melted down or otherwise destroyed.
Menorahs today typically have either six or seven branches because of some changes over time. During the time of the First and Second Temples, candelabra had seven branches. After the Temples were destroyed, which is said to have been around 586 BCE and 70 CE, respectively, a custom developed to preserve the sanctity of the Temples — not to reproduce things from the Temples. As a result, the six-branched version was born. In more modern times, some Jews have returned to using a seven branch menorah, however, arguing that even those aren't like the ones used in the Temples because they use electric- or candle-light rather than oil.
With both varieties, all but one of the lights are used for a purpose other than providing light. The last light, the shamash, is used both to provide light and to light the other candles. The seven-branched version is still found in many modern synagogues, but no longer serves any ritualistic purpose. It is rather kept for symbolic reasons, and in some synagogues, a simpler lamp is used in its stead. In fact, in terms of symbolism, it is the menorah, not the sign of David, that is the symbol of Judaism.
During the season of Hanukkah, the nine-branched menorah, or hanukiah, is used. The lighting of the Hanukkah menorah comes from a story in the Talmud in which the Maccabees were attempting to dedicate the Holy Temple. They were chagrined to find enough oil only for one evening, but went ahead with the dedication anyway and, miraculously, the small amount of oil lasted for a full eight days. The celebration of Hanukkah lasts for eight nights, with a new candle lit on each evening. The candles are lit from the left to the right.
There is some speculation that the original Menorah used straight branches rather than the curved ones often seen today. Some contemporary designs attempt to duplicate this appearance. Given visual representations on artifacts such as the Arch of Titus in Rome, various engravings, and coins released B.C.E, it is likely that the straight-branched candelabra is apocryphal and derived from a specific reading of Maimonides.