Like other Content Management Systems (CMS), Plone allows users to easily create and manage online content through the web by leveraging content types such as pages, blog posts, news items, calendar events, comments, etc. Those content types are typically custom-developed by programmers and require a formal code release to add the new types to a website.
And now Plone goes even farther: the Dexterity add-on allows non technical content contributors to not only use pre-existing content types, but to also define and create their own content types through the web (see screenshot).
Using Legos as an analogy, this means users can do more than just create a structure based on a given number of basic bricks: they can also design and use their own custom bricks.
Dexterity is Plone 4 compatible and the Six Feet Up development team is using it on most of our projects. It has several advantages over the classic "Archetype" content types, but the main improvement lies in the ability users now have to create content types in a WYSIWYG fashion. Many of our clients appreciate the fact that they can create their own content types, or tweak them on their own without having to ask for a developer's help every time they want to add or arrange a simple field.
Dexterity is also very developer friendly. For example, it is easy for a developer to export the content types created by a content contributor via Dexterity and reintegrate them into the code for optimal site efficiency. This also makes the content types data-independent and easily accessible to other web development projects.
With Dexterity, Plone now closes the feature gap with other well-known open source CMSs such as Wordpress and Drupal, which already offer this ability.