Real Ale is a beer that is served from the container that it was secondary fermented in (meaning that after the primary fermentation that made the beer is finished, it was placed in a vessel along with some form of sugar, the vessel is sealed allowing the resulting CO2 created as a byproduct of yeast interaction with the sugars to carbonate the beer). It is beer made from traditional ingredients, is served at temperatures between 50 and 55 degrees Fahrenheit (10C-12C) carbonated at the lower end of the spectrum 1.0,1.1 volumes. It is never dispensed by pushing it with CO2 (as kegged beer is).
It is usually not a big beer meaning that the alcohol level is between 2.x to 6 abv, there are some exceptions. Some of the finest real ales I’ve had were under 4 abv.
It is usually gravity dispensed from the cask or the cask is hooked to a hand pull otherwise known as a beer engine. When using a hand pull, the cask is out of sight under the bar or in a back refrigeration area. When gravity dispensed the cask can be on the bar or behind the bar area on racks.
The beer that comes out of the cask or beer engine is almost always clear, in-fact the English term is ‘dropped bright’ referring to the fact that the particulate matter has dropped out of the beer solution.
The clarity of the beer is not due to filtering. It is entirely from the correct cellaring of the beer, an action that most brewers and brewpubs have yet to understand in the US of A.