Stouts are stout, Pale Ales use pale malt, but where did the name porter come from?
There are a few theories out there, and their veracity, like that of most historical "facts", is hotly debated.
One theory begins in 1722 when Ralph Harwood, a London brewer, created a beer called Entire. For some time, working folk had been drinking a blend of beer, ale, and strong beer, which pubs would mix to balance out their stocks and maintain acceptable flavor.
Entire was a pre-mixed version, brewed by Hardwood then sold and delivered to pubs. Prior to this point, each pub had mixed their own. In addition to economies of scale, Harwood's method allowed for aging of the finished product. Historians theorize that the name porter came about because of the porters who worked at the markets and delivered the final product to the pubs. A variation of this theory suggests that porter simply refers to the working class occupations of those who drank the concoction.
Another theory suggests that porter comes from the Netherlands, where a beer called poorter was being made as early as the 1300s. The Dutch were prolific traders, and cultural transfer could easily have occurred. The term poorter originally concerned inhabitants of walled cities and may also have been related to social station.