Porter

History

Porter is a distinctly British Ale. In the early 1700s, the English Brew Pubs would pour up a blend of 3 beers to produce a hearty drink. This was especially popular among the dock workers.
In 1722, a brewer Ralph Harwood produced an Ale that had the characteristics of this blend, but was brewed to be served from a single tap. The beer was called 'Entire' to signify the entire drink was crafted in a single beer. Later the beer acquired the name 'Porter' based on the primary clientele.
The Ale was primarily derived from roasted brown or dark malt (rather than roasted barley), resulting in a roasted, malty, and even a smoky flavor. The wild "Bret" infection (using a Brettanomyces secondary fermentation) is a signature of the old style and is often described as smelling of horse sweat.
Porter marked the beginning of larger breweries and the demise of the smaller brew pubs. By the mid-1700s, to meet the growing popularity of Porter in England, Ireland, Scotland, and even the USA, larger scale brewing were necessary. Arthur Guinness and Sons claim to be the first commercial brewers of porter, and porter is considered to be the predecessor of Stout Porter and Stout. In fact, there is a fine (almost overlapping) line between the styles. By the way, Guinness stopped brewing porter in 1974.
Advances in malting technology produced paler malt with better extract yields. With the advent of the roasting machine in 1722, dark beers could be made cheaper using smaller quantities of roasted malt and a base of pale malt. This changed the character of Porter from the original beer brewed by Ralph Harwood, but closer resembles the modern renditions of Porter.
The popularity of Porter continued, but by the early 1900s the popularity of Porter was waning in favor other beer styles.
Modern commercial examples of Porter include: Anchor (a dark, roasted flavor), Sierra Nevada (with their distinct hoppiness), Redhook, Boulder, and U.K.'s Samuel Smith.

Characteristics

The characteristics of the style vary dramatically. The style varied naturally due to the wild assortment of adjuncts, herbs, and miscellaneous ingredients.

Statistics	O.G.: 1.040 .. 1.073, averaging at 1.050 F.G.: 1.010 .. 1.015 IBU: 20 .. 45 Alc.: 4.5 .. 6.1 % (by volume) Color: 25-35 SRM, ranging from reddish-brown to black
Water	Moderate to High in carbonate. pH: 3.9 .. 4.4
Malt	Bulk of Malt is Pale Malt Additional complexity, flavor & color from Crystal, Chocolate, and Black Malts Infusion mash at 150 .. 155 degrees (F)
Hops	Bittering Hops: Most any hops work fine Aromatic Hops: Golding, Cascade, Fuggles, or Williamette work well (Note: traditionally, all hops were added at the beginning of the boil. It is only the most modern porters that use aromatic hops).
Yeast	British Ale Yeast is best. (E.g. Liquid Wyeast 1098, or Dry Lallemand Nottingham). Ale Yeast is necessary for estery profile desired in porter. Ferment at 60 - 65 degrees (F) (Note: then there are the brave souls that may try secondary fermentation with Brettanomyces cultures)

Final Note: BJCP identifies 2 subcategories:

Robust, a dark ale with fairly full body, a coffe-like dryness from the use of roasted malts, a malty flavor, and a strong hop character for balance. Dark brown to black color.

Commercial Examples: Sierra Nevada Porter, Anchor Porter, Great Lakes' Edmund Fitgerald Porter (east), Summit Great Northern Porter (midwest)

Brown, generally lower alcohol than robust porter with less roasted malt character. Medium malt sweetness with low hop aroma and flavor. Medium light body. Medium to dark brown color. Starting gravities tend to be lower than robust porters.

Commercial Examples: Samuel Smith's Taddy Porter, Bateman's Salem Porter Shephard Neame Original Porte, Yuengling Porter (east)