That's a smoothing artifact, caused by the pronounced angle on the border of the glass. You can check Autosmooth in Mesh/Normals to get rid of it.
Note that Autosmmoth is a render trick (as well as Smooth shading), it doesn't actually modify the mesh. So for some cases, it won't work well. As an alternative, you can use a Edgesplit modifier instead : it will physically cut the edges, instead of simply adjusting the normals.