Thrust is an aerodynamic force that must be created by an airplane in order to overcome the drag (notice that thrust and drag act in opposite directions in the figure above). Airplanes create thrust using propellers, jet engines, or rockets. The propeller (or, on jet aircraft, the jets) provides the thrust that moves the plane forward.
A propeller is really just a special, spinning wing. If you look at the cross section of a propeller, you will find that a propeller has an airfoil shape and an angle of attack. Just by looking at the propeller pictured above, you can see that the angle of attack changes along the length of the propeller -- the angle is greater toward the center because the speed of the propeller through the air is slower close to the hub. Many larger propeller aircraft have more elaborate three-blade or four-blade props with adjustable pitch mechanisms. These mechanisms let the pilot adjust the propeller's angle of attack depending on air speed and altitude.