According to Old Testament, the Menorah was specifically designed by God with a central stem, representing His Spirit, with 3 stems on either side to represent the six attributes of wisdom, understanding, counsel, might, knowledge and fear. It is also said to represent the burning bush which Moses saw.
The Menorah is said to be the only symbol designed by God. In the Old Testament, Exodus chapter 25, Moses is given detailed instructions on what the lamp stand (the Menorah) should be made from and how it was to be made: "You shall make a lamp stand of pure gold...the whole of it one hammered piece of pure gold." As it represented God, it was to only be used in the temple.
While modern art shows the Menorah to have curved branches each side, Jewish writings such as the Talmud suggest the branches were straight, and diagonal to the central stem. It is thought to have symbolised a tree, in the same way the Torah was seen as "the Tree of Life".
There is a nine-branch version of the Menorah, called a Hanukkiah, which is used during Hanukkah - a traditional feast, though not one of those commanded in the Old Testament.
Interestingly, the Jewish historian Josephus, believed the seven lamps represented the seven “classical planets” in a set order: the moon, Mercury, Venus, the sun, Mars, Jupiter and Saturn.