Dexterity is a content type framework for CMF applications, with a particular emphasis on Plone. It can be viewed as more light-weight and modular alternative to Plone Archetypes content type framework. This is a comparatively new framework that uses technologies introduced by newer versions of CMF and Zope 3.
Dexterity was created to offer more abilities to Plone administrators/integrators and developers while dealing with content types in Plone. Such abilities as creating new content types through-the-web or switching on/off various aspects/behaviors on a per-type basis are now offered for administrators and integrators. Programmers in their turn can now create content types more quickly and easily than it was with Archetypes or plain CMF types.
Leveraging many technologies that come with newer versions of CMF and Zope 3, Dexterity contains less code than Archetypes; this allows better automated test coverage. Being more modular (compared to monolithic Archetypes) Dexterity allows easier support for things like SQL database-backed types, alternative workflow systems, instance-specific sub-types, etc.
With all the above taken into consideration, Dexterity is likely to form some part of Plone 4 and a more significant part of Plone 5.
More about Dexterity status and releases at http://docs.plone.org/4/en/develop/plone/content/dexterity.html