A Layer Mask is a one-to-one masking technique, i.e. a single mask is applied to a single layer. A clipping mask is more a one-to-many type method, as it can effect multiple layers at the same time. This tutorial explains and compares both.

Photoshop Masks preserve important pixel data and allow non-destructive editing techniques. There are five methods of masking in Photoshop: Quick Masks, Pixel Masks, Vector Masks, Clipping Masks and Clipping Paths. Learn which method to use to achieve clean, flexible and properly masked layers. photo credit: Juliana Coutinho