613 Mitzvot
Negative Mitzvah 44
Not to shave off the edges of your beard

The sages say this this mitzvah apply to using a razor to shave the beard. (Talmud Bavli Makkoth 20a) Others say that one should not even remove the beard with scissors. This is why many orthodox jews let their beard grow long.

We are not really told the reason for not shaving the edges of the beard, although the context might provide some clues.... There are three possibilities:

1. The shavings of the hair and beard might have been used in enchantments and sorcery, for the previous pasuk (verse) says, "neither shall you use enchantments, nor practice sorcery" (v. 26)
2. It might have been an idolatrous form of mourning, for the pasuk (verse) after this says, "You shall not make any cuttings in your flesh for the dead" (v.28).
3. This might be a part of a general prohibition against disfiguring our bodies, since we see we are not to cut our hair in a circle, destroy the edges of our beard, make cuttings in our flesh or have tatoos (v. 27,28).

"Jarchi and other Jewish writers say, there are five of them, two on the right, as Gersom reckons them, one on the upper jaw, the other on the nether, and two over against them on the left, and one in the place where the nether jaw joins the right to the left, the chin; the same observes, that it was the manner of idolaters to do the above things; and Maimonides (Moreh Nevochim, par. 3. c. 37. Hilchot Obede Cochabim, c. 12. sect. 1) is of opinion that the reason of the prohibition is, because the idolatrous priests used this custom; but this law does not respect priests only, but the people of Israel in general; wherefore rather it was occasioned by the Gentiles in common cutting their hair, in honour of their gods, as the Arabians did, as Herodotus relates, in imitation of Bacchus, and to the honour of him; and so with others, it was usual for young men to consecrate their hair to idols; but inasmuch as such practices were used on account of the dead, as Aben Ezra observes, it seems probable enough that these things are forbidden to be done on their account" (Gill)

According to Jamieson, Fausett and Brown, the ancient Egyptians trimmed their beards into a square form, which seemed to have some idolatrous meaning. However in the Bible we see that they Egyptians shaved their beards.

Bereshit (Genesis) 41:14
Then Par'oh sent and called Yosef, and they brought him hastily out of the dungeon. He shaved himself, changed his clothing, and came in to Par'oh.

2 Shemu'el (Samuel) 10:4,5
So Chanun took David's servants, and shaved off the one half of their beards, and cut off their garments in the middle, even to their buttocks, and sent them away. When they told it to David, he sent to meet them; for the men were greatly ashamed. The king said, Wait at Yericho until your beards be grown, and then return.

It was a shameful offense to go without a beard.

Vayikra (Leviticus) 14:9
It shall be on the seventh day, that he shall shave all his hair off his head and his beard and his eyebrows, even all his hair he shall shave off. He shall wash his clothes, and he shall bathe his body in water, then he shall be clean.

In the cleansing process of tzara'at, the beard was to be shaven off completely. Tzara'at is often translated "leprosy", but it is actually a spiritual disease that affects the skin.