Aircraft propellers are airfoils which convert rotary motion from a piston or turbine engine into thrust suitable for propelling an aircraft.
An aircraft propeller is a set of airfoils (most propellers have from two to six of these airfoils, which are called propeller blades) designed to convert the rotary motion of a spinning engine shaft (usually from a piston-engine or turboprop) into thrust which can be used to propel an aircraft.
All early aircraft used propellers for propulsion; small general-aviation aircraft and many commuter and regional airliners still use propellers, as do many midsize cargo aircraft and military transports, but they have been superseded by various types of jet-engine for essentially all combat aircraft and large airliners and cargo planes designed since the 1950s.
Aircraft propellers are generally made of metal, wood, or composite materials, and may be fixed-pitch, ground-adjustable, or constant-speed.
Additional information can be found in the Wikipedia article on aircraft propellers.