Prior to Plone 2.5, Plone was built on top of the powerful, but relatively inflexible, Zope 2 architecture. As Plone evolved, more flexible Zope 3 technologies became necessary, but a full transition was impractical. In order to remain compatible with earlier versions of Plone and provide a migration path, it was important to provide a bridge between these two versions. You might have heard the name "Five", which is a product that helped bridge the gap between Zope 2 and Zope 3 (2+3=5) by backporting Zope 3 capabilities to Zope 2. Five is now baked into Plone, and is the basis of the Plone 3 architecture. What this means is that Plone runs on Zope 2, but uses some of the features provided by Zope 3, and thus is not a pure Zope 3 implementation.
Zope 2 plus CMF (the Content Management Framework) is best described as a framework that uses skin layers and acquisition to produce results. If you know some of the concepts of object-oriented programming, you might think of...