If you have everything selected in the Default Cube, for instance, the Deleting Only Edges and Faces, will leave you with only the 8 Vertices. (You need to be in Vertex Select mode to see them!)
Only Faces will leave the Vertices and their connecting Edges.
There are cases, where some options do the same thing; when you have only one Face selected, for instance, Deleting Faces does the exact same thing as Deleting Only Faces.
