It can certainly be difficult to describe the difference between a lighter bodied stout and a porter... I would say that where a stout, like a dry stout in particular, has a very focused and intense roast character, a porter (and I am talking about 'robust porter' here) will have a round, slightly malty-sweet and open roast character that is considerably less intense... perhaps a good analogy would be the difference between a dark chocolate candy bar and a regular hershey bar. Both are vey chocolatey, but the dark chocolate is more intense. Then, added to the mix, you have the presence of noticeable hop flavor in a porter.