What's the difference between a Skin, Theme and a Package?

A Skin is just one look for squirrelmail, for example
W2:Iceblue is a "skin"

A Theme is a set of skins, for example:
Outlook XP Theme is a theme containing 4 skins

A Package is the archive you download, we provide several packages, the "base" package is a customized version of squirrelmail that you need to use for our skins to work. A skin package will either contain a single skin, or a theme.
A plugin package will contain a customized version of a plugin, designed to integrate into our skins.