In Vayikra 19:27, the Passuk says ולא תשחית את פאת זקנך - "and you shall not destroy the edge of your beard". I believe it says in Meseches Makkot (3:5) that this Passuk refers to 5 edges of the beard (or something to that effect).
The following picture, from this page which has since gone down, depicts all the opinions with pictures as to where exactly those five points are.
Translation of the Hebrew text:
Shulchan Aruch (Yoreh Deah), siman 181:11: The corners of the beard are five, and there are a multitude of opinions (as to what they are). Therefore one who fears heaven should fulfill all of them and not pass a razor upon his entire beard at all.
The images from left to right:
Leftmost:
Rashi in Shevuot in red; and the position of Rashi brought by the Rosh in Makkot in yellow.
Middle:
According to the Rambam
Rightmost:
According to Rabbenu Chananel in red, and according to the Rivan in yellow.
Rashi says in Parshas Kedoshim The end of the beard and its borders. And these are five: two on each cheek at the top [edge of the cheek] near the head, where [the cheek] is broad and has two “corners” [i.e., extremities, one near the temple and the other at the end of the cheek bone towards the center of the face]-and one below, on the chin, at the point where the two cheeks join together. - [Torath Kohanim 19: 74; Mak. 20b] Do with it what you want. There is plenty of discussion on the Beard from the Gemara to the Chofetz Chaim up to todays Poskim.
The corners of the beard are 1.) at the connection with the pe'ot, 2) at the base of the beard where the forks hang from the chin and 3.) the fifth one is the top edge and bottom edge of the beard, so this means dont cut any of it. It does not, however, include the mustache, which can be trimmed for sanitary purposes.
