Call sign's certainly aren't unique to the world of aviation, in fact, they are commonplace in most radio communications, and that is also where they are used in aviation. In aviation, the type of call sign is dependant on many factors. Call signs vary from the broadcaster- is he an aircraft or a ground station? The nature of the flight, and under which flight rules it operates also play a part.
For most light aircraft, their call sign is their aircraft type followed by the registration number. For example, a Cessna 172 with the registration N1234X would have the call sign Cessna One Two Three Four X-ray Usually, after initial contact with the air traffic control facility, the call sign is abbreviated, for easier and quicker transmissions.
Commercial operators, which include passenger and cargo airlines as well as air taxi providers have registered call signs that they use. These call signs are usually registered with the International Civil Aviation Organization (ICAO), which is an agency of the United Nations. Airline call signs can vary from the airline name, as is the case with United Airlines (United) or something more creative like an icon, for example, British Airway's Speed bird or US Airways' Cactus This call sign is used in conjunction with the flight number, therefore, US Airways flight 1 would have the call sign Cactus 1.
Performance and safety considerations also affect call signs. For example, wake turbulence, a type of turbulence formed from the vortices of heavy aircraft can pose a severe threat to aircraft operating in the area of an airport. To alert crews of smaller aircraft, most large aircraft include the call sign Heavy after their initial call sign. When an aircraft is in a state of emergency, it transmit's the call sign Mayday. Deriving from the French world for help, Mayday alerts controllers that the aircraft is in a dangerous situation and there is threat to human life, the aircraft and that which is under it. Pan Pan is one step below Mayday. It alerts the controllers that the aircraft would like to request priority, but it is not in danger.
Military aviators use different call altogether. They can use the call sign of their squadron, or their each and own personal call sign for internal communication.
Air ambulances and emergency aircraft use Lifeguard in their call sign, to specify that they usually need the shortest routes to their destinations.