Guess that means turboprops aren't considered jets?
Correct.
It has to do not with how combustion takes place in the engine, but how the engine provides propulsion "where the rubber meets the road". Turboprops use a stream of fluid (hot gas) to drive a turbine, but the turbine is not what pulls the plane forward; it's the prop.
Jet engines also have turbines inside, but again, they don't provide thrust- it's the fluid being expelled that does it.
I say "fluid" because it could be air, water, or almost anything that's a fluid: there are marine engines that work on the same principle... and there are even animals, like the squid, that use jet propulsion.
http://en.wikipedia.org/wiki/Jet_engine
http://en.wikipedia.org/wiki/Turboprop
In the case of the P-51 Mustang, it's neither a turboprop nor jet, but a piston-driven prop. Completely different sort of engine.
