MooTools 1.2 comes in two basic components: the MooTools Core and the MooTools Plugins (also sometimes referred to as "MooTools More").
The "Core" distribution includes everything covered up to this point in the MooTorial (Core, Class, Native, Element, Utilities, Fx, and Request).
The official plugins are things that the development team view as being useful but not necessarily stuff that you'll need in almost every application like you would the Core scripts.