This is a question I get a lot about stouts and porters, and the best answer I can give is that porters are slightly lighter and sweeter, while stouts are usually drier and heavier. Of course, there’s a hundred exceptions, from the breakfast-y sweetness of a milk stout to the boozy largess of a Baltic porter.
Which is why I’m forever thankful to the nerdlingers at GeekBeer for their detailed infographic about the differences between stouts and porters. They take the many classifications (dry stout, oatmeal stout, etc.) and map them on a two-dimensional axis of heavy/light and bitter/sweet. Then each point on the map is also drawn with an IBU range (international bitterness units) and a mini pie chart of a typical malt breakdown. It sounds complicated in words, but then again, that’s why they invented pretty pictures. Check out the chart here.