[vtkusers] Easy SetInput Question

[Willy]

Ok, here's an easy one from a supernoob.  In the two examples of VTK C++ code, 
what does the "(vtkDataSet *) mean?

vtkExtractVOI *evoi = vtkExtractVOI::New();
        evoi->SetInput((vtkDataSet *) PR->GetOutput());

 vtkImageGaussianSmooth *gauss = vtkImageGaussianSmooth::New();
        gauss->SetInput((vtkDataSet *) evoi->GetOutput());

Anybody want to walk me through these code snippets? I'm trying to learn C++, 
VTK, ITK, and OOP concepts, all at the same time.  It gets a little 
overwealming at times.  Thanks for your help.
Willy