Propellers and impellers both provide thrust, but do it in different ways.

A propeller is a fan which propels a fluid by pushing against it: it converts rotational motion into linear motion. An impeller is a rotor that produces a sucking force, and is part of a pump. A propeller is always “open” and an impeller is always “closed” (as it has to draw fluid into something).

L-R: Impeller and propeller