A net will always be made up of the same number of connected 2D shapes as there are faces on the 3D shape, excluding folding flaps of course. For example, this cube net becomes this die.

Cubes, cuboids, tetrahedron and prisms are examples of polyhedra. When you open out a polyhedron as a net, it shows all the polygons that it is made from. For example here is the net of a cuboid.