OSG stands for OpenSceneGraph and is an open source high peformance 3D graphics toolkit (http://www.openscenegraph.org). A documentation can be found on their webpage under documentation or by klicking the link http://www.openscenegraph.org/osgwiki/pmwiki.php/Documentation/Documentation
For now only the primitives should be explained, which are required to build new objects for the simulations:
The mentioned primitves are drivatives of lpzrobots::Primitive which is an interface class for primitives represented in the physical and graphical world. It is intended to bring OSG and ODE together and hide most implementation details.
For visualization the primitves use the osgprimitives (since this is done in the primitives you don't have to bother about this one):
They are derived from lpzrobots::OSGPrimitive which is an interface class for graphic primitives like spheres, boxes, and meshes, which can be drawn by OSG.