The basic concept is that as a gas moves from an area of high pressure to an area of low pressure, it gets colder (high pressure = high temp, and low pressure = low temp). Your AC has freon at high pressure in a can, it uses electricity to force this freon through a tiny hole into a larger area, thereby dropping the pressure and temperature rapidly. Fans then blow this cold air out into your house.
Freon is becomming less common as a refridgerant, since it could be harmful to the environment.
Its a little more complicated, but that is the basics of it.