Diem is a content management framework (CMF) for PHP projects.
As a framework,
Diem is flexible. There is no preinstalled stuff ( blog, comments... ) : the project starts empty. The developer creates its own modules by declaring them in configuration files. Then Diem generates a code which is 100% specific to the site's needs.
As a CMS,
Diem packages many site-independant features in a clean user interface. Most of the common problems a web developer has to face up are made easy.
Diem is, and will always be, entirely free and published under the MIT license.
Please see to the online documentation : Diem documentation