Aircraft

Unlike many people think, an aircraft is not just the common commercial airliner but any vehicle capable of flying controllably in the air. Aircraft can be categorized according to different classifications. The major ones are the following:

Propulsion
According to their method of propulsion, which is the most common classification system, aircraft can be classified as being unpowered (such as gliders, balloons, and kites) or powered (such as helicopters, propeller aircraft and jets).

An unpowered aircraft, the Schleicher ASK 21 glider (image embedded from Wikipedia on 19 May 2009)	A powered aircraft, the HAL Dhruv helicopter (image embedded from Wikipedia on 19 May 2009)
Lift
According to the lifting force that allows them to fly, aircraft can be classified as aerostats or aerodynes. Aerostasts (such as balloons and dirigibles) are lighter than air and, therefore, "float"¹. Aerodynes (such as airplanes and helicopters), on the contrary, are heavier than air and need both wings and a method to exert some sort of force to move air over those wings in order for them to gain lift².

An aerostat, the American Blimp Corporation A60 (image embedded from Wikipedia on 19 May 2009)	An aerodyne, the Boeing 747SP (image embedded from Wikipedia on 19 May 2009)
Use
According to their operational use, aircraft can be classified as military, civil, experimental and model aircraft.

A military aircraft, the A-10 Thunderbolt (image embedded from Wikipedia on 19 May 2009)	A civil aircraft, the Airbus A380 (image embedded from Wikipedia on 19 May 2009)
Manning
According to whether the aircraft requires a pilot or can be controlled remotely, aircraft can be classified as manned or unmanned. The typical aircraft is manned in-situ, i.e. it is flown by one pilot or a crew. However, unmanned aircraft -those without pilots- are becoming more common, especially for military and research purposes.

A manned aircraft, the Saab Gripen figther (image embedded from Wikipedia on 20 May 2009)	An unmanned aircraft (or unmanned aerial vehicle), the MQ-9 Reaper (image embedded from Wikipedia on 20 May 2009)
Speed
According to their average flight speed, aircraft can be classified as supersonic or subsonic aircraft, depending whether they can travel at speeds higher than the speed of sound or not, respectively.

A subsonic aircraft, the Boeing 737-300 (image embedded from Wikipedia on 29 March 2010)	A supersonic aircraft, the FA-18 Hornet approaching the sound barrier (image embedded from Wikipedia on 29 March 2010)