The fluid responsible for allowing your engine to turn your automatic transmission so that you can move your vehicle when you slide the gearshift lever into the D position is called the transmission fluid! A lot of people don't realize how much stress is put on this fluid and how temperature extremes wear down this stuff significantly. The going rate for this service is approximately $139 and is worth every penny!
The automatic transmission is a modern marvel of how hydraulics work. Fluid is pushed through many valves, a filter, a torque converter, and variable rates of pressure to control what they want when they want it. The engine actually spins a part in the torque converter, which in turn pushes fluid against a different part in the torque converter, to actually transfer the power from the engine to the transmission. That's why we don't have to push a clutch! It's other responsibilities include lubricating the gears and clutches and cooling the transmission. So transmissions are what they call transaxles, because they contain a differential assembly as well. That means the fluid also has to lubricate those gears.
That is why the average interval for most vehicles is 30K miles for each fluid change. Some manufactures recommend longer intervals, such as 50K, 60K, or 100K miles changes. There are so many different types of transmission fluids out there, and acceptable use generally has a lot to do with what the manufacturer recommends.
For example, there is Dextron for GM's, Mercon for Fords, and ATF+ for Chrysler products. What makes this even more confusing is that most of those have numbers after them (example is Mercon IV) that signifys what standard they are meeting. Most are backwards compatible. This is just the tip of the iceberg, as there are so many more different types of transmission fluids. There are universal fluids out there that meet multiple standards of fluids.
Sometimes the manufacturer also recommends that the filter be changed as well. Most shops will tackle this light duty transmission service, but you may have to take it to a transmission shop. A few cars have a spin on transmission filter, but most are accessed by draining the fluid and removing the transmission pan. If your car has one, the heat exchanger (cooler) needs to be flushed out as well. Some trucks have external coolers when they include a tow package, but most coolers are located inside of the radiator assembly. They are, of course, separated from the engine coolant. A busted radiator can actually leak coolant into the transmission or vice versa.
A few hundred dollars every year or two can save you a 4K dollar transmission replacement! Don't forget to service your other drivetrain components, like differentials and transfer cases.