Layer comp definition from Photoshop CS2 help:
V 1.90 <x>A layer comp is a snapshot of a state of the Layers palette. Layer comps record three types of layer options:
Layer visibility—whether a layer is showing or hidden.
Layer position in the document.
Layer appearance—whether a layer style is applied to the layer and the layer’s blending mode.
Using layer comps, you can create, manage, and view multiple versions of a layout in a single Photoshop or ImageReady file.