I'm experimenting with VDBFracture. For my cutter object I have a sphere and it's doing an intersecting boolean type effect, after reading the docs on this node, is this sorta the purpose of the node ?

Also how do you use groups, the docs mention you have to specify a volume but what attributes can I use, besides density ?

Take a look at the Houdini Examples over here.
http://www.openvdb.org/download/ [openvdb.org]

The hip file is pretty well documented.

I've taken a look at the examples but they don't explain how and why for some VDB nodes as in VDB Fracture.

I understood VDB Fracture but does anyone know any tutorials besides the example file which is on the official OpenVDB site that explains the nuts and bolts of most, if not all of the OpenVDB nodes to get a clearer understanding ?