How does deep frying work?

Take a look at the sources I linked. They answer your questions. In essence, deep frying allows one to cook certain foods much faster than baking or broiling, at a higher temperature to allow for a crisp surface on the outside but with a soft, moist inside.

The oil is much more conductive of heat than air in a convection oven. That is why it can heat the food faster. At higher temperatures the browning and crispiness is due to melting and possibly partial decomposition of the sugars causing a browning affect, known as carmelization. The sugars may also react with amino acids to cause interesting flavors and a browning affect as well by undergoing what is known as the Malliard reaction.

Well fat tastes good and oil is basically a whole lot of fat. From the perspective of chemistry, there is a whole lot of energy absorbed by the food being fryed...