The AGA is the original heat storage range cooker.
Developed and invented when a more efficient method of cooking and roomheating was needed to replace the open-fired range. By enclosing the heat source (originally an open solid-fuel fire) it was made much more efficient and much more controllable. The layout also allowed for two hot-plates to be added - a far better solution than hanging a kettle over the open coals or on a grid shelf above the firebox.
The AGA is heated from one central source (originally a solid fuel fire but now more likely to be oil, gas or electric) and the cast iron components (hot-plates, top, front and ovens) store the heat, distributing it in the correct proportions to each oven and hot-plate, emitting a gentle warmth to the room too.
The AGA stores heat in the cast iron hot-plates and ovens, since they take time to heat up (or cool down) the cooker is maintained at its normal running temperature all the time (unless you're using a newer 'programmable AGA' - see the separate article).
Let's get a little scientific: to cook something the result is made up of time and heat; the higher the heat, the quicker it cooks. We all know that you can speed things up by using a higher temperature but you can't get away with that everytime as some larger items will burn on the outside before they're cooked on the inside so recipes specify an oven temperature and a cooking time.
Cooking in an 'ordinary' electric or gas oven you'd pre-heat to the required temperature and then cook for the alotted time, generally using the same position in the oven. The 'variable' in this 'cooking equation' is the oven temperature.
Using the AGA you simply use the oven that's set at the temperature you need (no need to pre-heat). If you want to roast - use the roasting oven, if you want to bake - use the baking oven, simmer - you guessed it - the simmering oven. Cooking times don't really change and neither do the temperatures, it's just that you put the food in the oven that suits the required temperature.
AGA ovens aren't fan ovens so are hotter towards the top - you can put your food higher or lower within an oven to vary the heat.
The heat in the roasting oven is radiated from all round, not one specific place, giving an even temperature. (You do still need to turn things round part way through like in any oven though.) Since the oven is hotter at the top you get a grilling effect if you put food high up - perfect for cheese on toast, or finishing the Shepherd's pie!
There's a hot-plate effect from the floor of the oven too - ideal for pastry, no need to blind bake as the floor of the oven ensures the pastry is fully cooked and avoiding the dreaded 'soggy bottom'!
The cast iron ovens then cook without an exposed element or flame and give the unique even heat that cooks food so wonderfully - retaining moisture and cooking through without drying out.
Another benefit to cooking on the AGA rather than an 'ordinary' oven and hob is that you make much more use of the ovens. Simmering vegetables - Instead of boiling the colour, flavour and nutrients out of your veg on the hob just bring to the boil on the hot plate, drain the water from the pan and put the lid on then steam in the simmering oven for twice the remaining time. It'll save heat and make things taste so much better.
The AGA's ovens are vented to outside (most models) so you can cook different foods at the same time without flavours transferring. If you're steaming rice or veg the steam won't cause condensation in the kitchen and you won't smell the kippers until they come out of the oven.