An AGA cooker is a type of oven that has been popular in the United Kingdom since the 1930s, although their popularity has waned in recent decades. Calling an AGA an oven is misleading, however, as they work on the principle of maintaining a constant, low level heat source to a thermal mass of heavy cast iron. The result is that they are often multipurpose; in addition to oven and stove top, an AGA may also function as a water heater and an effective space heater.
AGAs were originally designed to work on slow-burning coal, and were popularly used in large country houses and farm houses. They are somewhat iconic of upper middle class English life, and are famously wonderful ovens for baking. They are also famous for eating an enormous amount of fuel and taking six hours to warm up properly. They were traditionally left running 24 hours a day, providing a useful heat source during the winter... and an obnoxious heat source during the summer.
The current AGA models are a bit more refined -- which is not to say that they are comparable to other models of cookers; they are still one of the less fuel-efficient and more expensive cookers on the market, as they work by having a heating element applied to heavy cast-iron heating plates, which in turn radiate the heat into the oven. With the assistance of electric heating coils the modern oven takes about 35 minutes to heat up, while the hotplates on the stove-top take almost 10 minutes to warm up. They are modern however, with the most popular model today being an electric version that is controlled by touch-screen, and by early 2016 there will be an app so that you can control your cooker remotely by iPhone. Even the most modern models maintain the classic big-boned, cast-iron aesthetic.
AGA is a bastardized abbreviation of the Swedish name Aktiebolaget Svenska Gasaccumulator, which is commonly half-translated into English as 'Aktiebolaget Gas Accumulator'. AGA is pronounced as if it were a standard word, 'aga', although when pronounced with the soft intrusive R in many British accents, often sounds like 'augar' to American ears.