Mysite PDF on Plone, Zope and the CMF has a useful description on page 14 of the role of layers and skins:

This is a concept to extend name/method look up that the CMF adds to Zope. I like to think of this as having multiple
different search paths to choose from. Suppose you have a number of folders (or layers as they are called in the
context of skinning) f1, f2, ..., fn holding different versions of the same set of methods (but there is no need to
have each method in each folder). Now you define a skin by saying: for skin A first look in f1 , then look in f2 , then
in f3 whereas skin B might be defined as: first look in f2 , then in f3 , then in f1 and so on. That way you can
provide, e.g., different designs to choose from (for example by having different style sheets in the different folders).
But the concept is more powerful than to just provide different designs. In principle you can use it to provide multiple
object behaviour , meaning that you can make objects behave differently in response to particular method calls if the
method is implemented to do different things in the different skin folders.
Under the hood, the skins tool works by extending the acquisition context of all objects within a CMF site. But you
cannot tell this from looking at the URL in your browser, because this is done "silently" when traversing a so-called
SkinnableObjectManager , a class that the PloneSite class itself inherits from.

There is also a useful discussion from Andy McKay's book, referenced here, on skins and layers.