Astronomy: what is the difference between a trojan and a moon?

They are entirely different criteria and a celestial body could actually be both.

A moon is any body that orbits a planet or dwarf planet. (Some authors include bodies that orbit asteroids below darf planet size as well)

A trojan is a body that moves on the same orbit as a larger body, either 60 degrees ahead or behind the other one. These points are stable - small disturbances of such orbits will correct themselves.

A trojan can also be a moon, if it (as well as a larger moon) orbit a planet.

https://en.wikipedia.org/wiki/Trojan_(celestial_body)
https://en.wikipedia.org/wiki/Natural_satellite