Rim lighting is a common technique used in photography and computer graphics. The technique puts a light source on the opposite side of the object and away from the camera. This positioning is used to highlight the silhouette of the object and to better separate it from the background.
The sample also provides a simple arcball class and a ModelViewerCamera class. The arcball class handles rotations used internally in ModelViewerCamera class. ModelViewerCamera actually uses two arcballs: one for rotating the object in the World, and one for rotating the camera around the object. When the camera is rotated around the object, its position changes, but the camera is always looking at the object. This is similar to a satellite circling earth, while its antenna always faces the earth.