Try as you might, you can't define the porter. This style has gone through so many transformations over the years that it's nearly impossible to say "a porter is this, and a porter is that." Loosely defined, a porter is a dark, full-bodied ale that's a little more roasty than a brown ale, but less than a stout, and overlaps a lot of that of the dark mild. Make sense? No matter if it spotlights the Great English History or the hazlenut, the porter is an incredibly versatile beer that is revered for its food-pairing potential and drinkability the world-over.
The only thing definitive about the porter is that the English do the original and do it best; all others merely expand upon the style with their own tastes. Porters from The Kernel, Harviestoun, and Otley Brewing show exactly what we mean. With that said, an American porter is a nice interpretation and are a little more assertive and robust; try the Anchor Porter or the Rogue Mocha Porter for a taste. One cool thing about the porter is that it's really good for showcasing interesting ingredients: smoked malts, chili, chocolate. Mikkeller and Saltaire each have a fabulous example. Long live the porter! Whatever it is.