Simply put, VTK is an object-oriented approach to 3D graphics and Visualization.
The 3D graphics model is at a higher-level of abstraction than other common rendering libraries such as openGL.
The Visualization model supports virtually all common visualization algorithms.

This image used to rotate, but is now static to conserve space

The code for the rotating ?, above

#include "vtk.hh"

main() {

  int i;

  // These are incredibly typical things-
  // we'll use them virtually everytime we write programs.

  vtkRenderMaster  rm;
  vtkRenderWindow *rw;
  vtkRenderer     *ren;

  // These declarations will vary depending
  // on what we are trying to do.
  vtkTextSource   *string;
  vtkPolyMapper   *polyMapper;
  vtkActor        *actor;

  // First we make our window.
  rw   = rm.MakeRenderWindow();
  ren  = rw->MakeRenderer();

  // Next we create a bunch of polygons to
  // represent our question mark.
  string = new vtkTextSource;

  // Now, we send those polygons out to be mapped.
  polyMapper = new vtkPolyMapper;

  // The only actor in our scene will be comprised
  // of this group of polygons.
  actor = new vtkActor;

  // Add the actor and render the scene!

  // Animating isn't all that tough, either.
  for(i=0;i<12;i++) {