Sentence:  Just as there are only five <WSD>regular maps</WSD> on the sphere (or plane), there are only "three" classes of <WSD>regular maps</WSD> that can be created on a torus. 
Meaning of the word marked with <WSD> tags:  A symmetric tessellation of a closed surface; a decomposition of a two-dimensional manifold into topological disks such that every flag (incident vertex-edge-face triple) can be transformed into any other flag by a symmetry (i.e., an automorphism) of the decomposition.