According to this article by Tzomet, it is forbidden for Jewish males to shave their beards with a razor; however, most electric shavers (with the possible exception of "lift and cut" shavers) are allowed to be used.
What about a manual hair clipper?
This came up in an answer over at Lifehacks, about alternative shavers for when an electric shaver breaks or otherwise ceases to function.
The same logic used with an electric shaver would apply here. If it's not "razor-sharp", it's fine.
There are a few different definitions; Rabbi Heinemann of the Star-K, shlit'a, in the name of R' Moshe Feinstein suggests taking a whisker from someone's beard, and seeing whether the cutter can cut it the same way a razor would.
Hagaon R' Moshe Feinstein זצ"ל , who permitted the use of electric shavers used a criteria similar to the shochet who would demonstrate how sharp his חלף (shechita knife) was. To show the sharp edge of his knife, a shochet would take a hair from his beard, and holding the hair in one hand the shochet would see whether the חלף cut the dangling hair. If the hair was severed, the shochet's steel passed the test.
My understanding is that a manual clipper works like a fancy set of scissors, is not "razor" sharp, and is therefore permissible.
