How does a propeller work?

A propeller works by converting the rotation of the engine into horizontal thrust similar to a fan. The blades push the air backwards creating acceleration, and create a difference in pressure between the forward and rear surface adding to the movement.

The above is, of course true for a boat propeller. For an airplane propeller Bernoulli's pincipal is involved. Bernoulli's principle states that the greater the speed of a fluid, the less the lateral pressure. So the propeller uses the same deal as the wing.