My van has three on the ceiling and two on each rear wall. They appear to be asphalt based pads which I think is what the original Dynamat products where made from. The front doors also have small triangle shaped pads that are butyl instead of asphalt and aren't much bigger than a Dorito chip. Butyl caulk or tape is also what's used to secure the plastic dust shields inside the doors.
The pads work on the premise that securing to even a small area on a body panel can greatly reduce the resonance which makes the vehicle quieter. You can test the effect yourself by tapping on a panel without the pad and then tapping on a panel that includes one.
The Sprinter vans I've seen use a lot more and basically have a pad stuck to every section of the roof.