I have heard that it is permissible according to Halacha to shave ones neck using a razor. Is this correct and if so where are the boundaries?

It is forbidden to shave one’s beard or sideburns (the stretch of hair connecting the head with the cheekbones on either side of the face) with a razor. The prohibition includes the area of the throat until the Adam’s-apple. It is permitted to shave the back of one’s neck (at the back of the sideburns, from behind the top of the ears) with a razor (Shulchan Aruch and Rema Yoreh Deah 181).