They do very extensive cleaning that only a few people actually do to their cars because it takes too long (or they just don't want to do it themselves), they do everything from just interior/exterior cleaning to leather/upholstery/paint/headlamp restoration. The price depends on a vehicle size and the services requested. They also do "ionizing" (I think that's what it's called) when they shoot some stuff through the A/C system and let it cycle in the car where it kills all the smells by killing the bacteria that causes it.

Some shops give you a loaner car to drive while they do it, it's usually covered with advertisements for the place that's detailing your car. That's their "free" advertisement".