Plone has been called Zope's killer app. Out of the box, it provides a usable, flexible content management system. Its real strength, however, is as a framework for CMS construction.
Plone itself builds on Zope's CMF (Content Management Framework), which was developed by Zope Corporation and originally intended to fill the role that Plone eventually filled. The crucial difference is that Plone took the visual design and user interface aspects very seriously. Whereas a plain CMF site needs a lot of attention to make something presentable of it, Plone comes with an interface, content types and configuration options that make it immediately useful for a personal portal-type site.
As the Plone project gained momentum, another aspect that began to distinguish it is the establishment of conventions and mechanisms for customization and extension. Plone has added many portal tools to those provided by the CMF, and the underlying principles of its user interface (skins, portlets and actions) have proven very flexible and reusable.
Plone is being used in a number of high-volume sites by organisations such as Lufthansa, NASA, Rice University and the British Strategic Rail Authority. For a longer list, see Plone Sites.