Appearance: Very dark, almost black in color. Thick head, creamy in texture and light tan to brown in color, which will last while drinking the beer. (Think of the mustache you get while drinking a Guinness.)
Aroma: Roasted malt and barley lend a coffee like character to the aroma. You may get a hint of cocoa, but that's not necessary. European hop aroma at very low levels, also ok if not perceived at all.
Flavor: Like the aroma, there will be very low to no European hop flavor. Light caramel flavor is common. The finish will be dry and roasty due to the roasted barley. The bitterness comes more from the roasted character than the hops used.
Mouthfeel: Body should be somewhere between medium-light and medium full- much lighter than the color and head would lead you to believe if you've never had it before. It should be very smooth, with low carbonation.
Examples: Guinness, Murphy's, Sun King Ring of Dingle, Brooklyn Dry Stout
In order to get a better idea for the style, and not be too biased in my representation, I looked at a few sets of guidelines. Links are below in case you're interested in delving in a bit more.
- BCJP Style Guidelines: http://www.bjcp.org/stylecenter.php
- Beer Advocate: http://www.beeradvocate.com/beer/style/162/
- Brewers Association: http://www.brewersassociation.org/pages/business-tools/publications/beer-style-guidelines
- GABF Style Guidelines: http://www.greatamericanbeerfestival.com/the-competition/beer-styles/