What's the difference between a porter and a stout? Porters and stouts seem very similar and sometimes stores and restaurants group them together as if they're two different names for the same thing.

Is there a difference and what is it?
Beer Connoisseur Online answers this pretty thoroughly in an article.  

The TLDR: is that "...originally a stout was a strong version of a porter.  Today, the difference is whatever you want it to be."

For an interesting history on the term Stout and the style of beer, you can check out this history of stout article provided by Eric Deloak.  Similarly you can read a history of porter article written by the same author: Gregg Smith