what is the difference between hydralic and pneumatic system

Pneumatic system is operated by compressed air while hydraulic system is operated with pressurised hydraulic oil
or any type viscous oil.
Hydraulic systems can usually produce higher control forces and work under extreme
operating conditions. This is the system that is primarily used on aircraft to move
control surfaces and engine controls.
You can often classify pneumatic systems because they have a pump (gas pump) and
hydraulic systems have a piston.
That is the difference. Also, pneumatic systems are louder because it is a gas under pressure.

1. By definition, hydraulics is used in controlling or harnessing power
with the use of pressurized fluids whereas pneumatics studies how pressurized
gases influences mechanical motion or movement.
2. Hydraulics uses an incompressible fluid medium like oil whereas pneumatics
uses a compressible gas like air.
3. Hydraulic applications demand greater pressures during operations that reach
thousands of pounds per square inch whereas pneumatic applications only
require 100 psi pressures more or less.
4. Most hydraulic applications generally use bigger components that pneumatic applications.
5. Hydraulic systems are generally more difficult to operate compared to pneumatic applications