In season 2 of Tokyo Ghoul at least a couple of times is mentioned classes of Ghouls. In one chapter one person talks about "Ghouls from class C to SS" IIRC. Which are all the classes of Ghouls?
Rating starts from C and goes on to SSS. However, classes not only meant to show how powerful the ghoul is but it's more like a "how many and which class investigator we must send" measurement.
If you want to see who fall into what classes you can visit here.