The aircraft registration number, often called the tail number, is to civil aircraft as a license plate is to land vehicles. It can help you buy a plane wisely.
An article in Aerospaceweb.org provides detailed information about aircraft registration numbers. The need for this type of civil aircraft identification came out of the 1919 Paris Peace Conference. The intent of these gatherings was to create an international alliance for regulating civil aircraft use. This need arose primarily as nations were starting to expand their civilian aircraft fleets. Among the agreements formed by the global aviation community was the systematic way of aircraft registration and identification, as governed by the International Civil Aviation Organization (ICAO).