World's most popular and powerful Content Management System - Plone is built on top of Zope web application server and its Content Management Framework. Zope is an open source web application server, written in the Python programming language. It is a scalable, stable, powerful system that includes an object database, a web server and several templating languages.
Zope is a framework for building content management software, i.e Zope is an operating system for web applications, one of which is CMF (Content Management Framework), an application to facilitate the building of CMSes. Plone is one of such CMSs, running on Zope, but with its own set of templates and file types. Zope provides Plone with many outstanding features and abilities that make it is easy to customize and expand Plone.
Since Plone runs atop the Zope web-application server, that is written primarily in Python with some C language components, a working installation of Plone requires Plone, Zope and Python. Only a package of these three core elements that are set up correctly can ensure correct work of any Plone project.
Both Plone and Zope are Open Source software, which means that their source code is available to anyone for free. There are many reasons why people prefer Plone and Zope rather than other open source CMSs.
See more information about advantages of Plone Zope in the What is Zope? article.