Differences between Stouts

The short answer is that oatmeal is used in the brewing process which produces a more mellow taste characteristic and heavier mouthfeel in the beer, and many times a more long-lasting head. Here's a link to the BA style definition page where you can go to the Stouts section and read the definition, characteristics, etc. https://www.beeradvocate.com/beer/styles/

 

I should have been a little more informative in the way I worded the part about oatmeal being used in the brewing process. I did not mean to say or imply that oatmeal is the only grain. Barley is still the main malted grain that is used to brew the beer, but the oatmeal is added to the grain bill for this beer style in addition to other grains that are typically used in a stout.