What is the Difference Between a Hydraulic and Air Bag Suspension?
There are two main types of ride height adjusting systems, air bag or pneumatic, and hydraulic suspension systems. Air bag systems use a bag enclosed in a heavy rubber case, and a series of valves connected to a compressor to adjust ride quality and height. The valves control the amount of air pumped into each bag, effectively raising and lowering the vehicle. Hydraulic suspensions utilize hydraulic fluid and shocks or struts instead of air bags and air pressure. Hydraulic suspensions give a much stiffer ride because hydraulic fluid isn’t compressible the way air is. They also raise and lower much quicker for the same reason.
For more info, see Air Suspension Bag