A gas turbine engine is basically a heat engine using air as a fluid to produce thrust. The working principle of the gas turbine engine is comparable to that of a piston engine as both engines use a cycle made of induction, compression, combustion, and exhaust phases.
The only difference is the way these phases will be achieved as the gas turbine engine is capable of dealing with much larger amounts of energy for a given size and weight. The other great advantage of the gas turbine engine is that the whole working cycle is continuous and entirely rotational providing an overall smoother operation, whereas the piston engine uses an intermittent reciprocating motion which is converted to rotary motion by means of cranks.

Summary:
Piston engine
-Process is intermittent
-Combustion is at constant volume

Jet engine
-Process is continuous
-Combustion is at constant pressure.