At the moment, the development of OpenFOAM have been forked into three main development:

• OpenFOAM+ (OpenCFD Ltd, ESI Group)
• OpenFOAM (The OpenFOAM Foundation Ltd.)
• Foam-Extend (Wikki Ltd.)

Although users may find specialised features (solvers/utilities) available in one version but not in the others, all versions offer same core solvers and functionalities, Beginning users can expect that general usage of these different versions are similar, and using different versions interchangeably depending on specific requirements is straightforward after becoming familiar with one. It is a good practise to take note on a specific version and version number used when referring available resources online.

A comprehensive discussion on the choices between the main versions can be found here: ‘OpenFOAM.com versus OpenFOAM.org: Which Version to Use? -- CFD Online Discussion Forums’.

Additional forks are described ¹⁾ in the following page: Forks and Variants - OpenFOAMWiki.