The Harrier, informally referred to as the Jump Jet, is a family of British-designed military jet aircraft capable of vertical/short takeoff and landing (V/STOL) operations. The Harrier family is the only truly successful design of this type from the many that arose in the 1960s.
There are four main versions of the Harrier family: Hawker Siddeley Harrier, British Aerospace Sea Harrier, Boeing/BAE Systems AV-8B Harrier II, and BAE Systems/Boeing Harrier II. The Hawker Siddeley Harrier is the first generation-version and is also known as the AV-8A Harrier. The Sea Harrier is a naval strike/air defence fighter. The AV-8B and BAE Harrier II are the US and British variants respectively of the second generation Harrier aircraft.
Historically the Harrier was developed to operate from ad-hoc facilities such as car parks or forest clearings, avoiding the need for large air bases vulnerable to tactical nuclear weapons. Later the design was adapted for use from aircraft carriers.
Source: Wikipedia, "Harrier Jump Jet", available under the CC-BY-SA License.