A runway is a "defined rectangular area on a land aerodrome prepared for the landing and takeoff of aircraft" (ICAO definition).
A runway is the long, rectangular surface area that aircraft land on and take off from. Larger runways are usually constructed entirely from artificial materials such as asphalt or concrete, but runways are also made by preparing natural surfaces such as grass, dirt, snow or even ice. The term "water runway" is sometimes used in seaplane operations, but a runway is normally understood to be on land.
Most runways have markings to provide information about the runway heading, touchdown zone, surface available for landings and so on. Additionally, many runways have lighting for use at night or in low visibility. Smaller runways in remote locations may have no markings or lighting of any kind.
Runways are orientated depending on the prevailing winds (aircraft almost always take off and land into the wind) and other factors such as terrain. They are numbered based on their magnetic heading: "runway 18/36" describes a north-south runway, referred to as "runway three six" when taking off to the north and "runway one eight" when taking off to the south. Parallel runways at the same airport usually have the same number with "left" and "right" appended, e.g. "one eight left" and "three six right" would refer to the same physical runway. If there are three parallel runways, they are designated "left", "center" and "right".
The "active runway" is the runway currently in use for takeoffs and landings; a large airport may have several active runways at once. The active runway is primarily determined by the current wind direction and can change at any time. At controlled airports this is managed by Air Traffic Control but at uncontrolled airports pilots decide themselves which runway to use and communicate with other pilots appropriately.