Mudbox's 3D Layers behave much like 2D layer systems of mainstream image editors such as Photoshop®. When sculpting on a layer, Mudbox calculates the difference between the positions of vertices after a brush stroke and their original positions on the "base" mesh. Only this difference (the delta) is stored on the layer. This delta is added to the base mesh and to the deltas stored in all the other layers to arrive at the final shape of the mesh.
A: menu, B: visibility toggle,
C: multiplier, D: Layer Window Lock,
E: active (highlighted) layer, F: base mesh, G: base mesh
properties
The Layers window 
menu contains all the commands for managing layers.
Create layers using the New Layer command. New layers are inserted
directly above the current active layer. There is currently no way
to reorder layers. However, since the layers combine additively,
the order of layers is not important to the final result.
Each layer has a multiplier value that can be edited through a number field or slider. By default the multiplier ranges from 0 to 100; however, values outside of this range can be inputted to amplify or invert the information stored on the layer.
[PICTURE: INVERTING AND AMPLIFYING DETAIL]
Layers and Levels
A layer is automatically locked to the level on which it is first edited. From then on the layer can be edited only on that level.