The Plone CMS is a direct offshoot from Zope platform. More than that, it has become an important driver -- perhaps the most important driver of Zope adoption.
Zope is an open source platform for building content management systems, intranets, portals, and custom applications. The Zope community consists of hundreds of companies and thousands of developers all over the world, working on building the platform and Zope applications. Zope is written in Python, a highly-productive, object-oriented scripting language.
One of the major benefits of Plone is that it runs on just about anything (Windows, Linux, Mac OS X, FreeBSD, Solaris, etc.).
Unlike many other CM systems, especially commercial ones, Plone’s administrative interface works with just about any web browser. It works best with modern browsers, but you’ll get by with an older one if you need to. In other words, using Plone will not force you to use Internet Explorer for Windows. That is a major benefit.