Callsigns
Proper callsigns are allocated by ICAO and are valid globally. They are a callsign (preferably something that is not likely to be misheard on the radio) and an associated 2, or 3, letter identifier. The identifier is used, with a number, on the flightplan instead of a tail number. Any person, or organisation, with a good reason for needing a callsign can apply for one. Airlines use a callsign and flight number so that a specific route/timing always has the same identity whatever physical aircraft is being flown (they may not know this at time of filing). Others may use a number to identify the pilot, the purpose or the date of flight.
Due to the large numbers of applicants there are now restricted callsigns in use; they tend to be valid in specific areas and do not have an associated identifier. They sound good and mean you don't need to search for the tail number every time you open your mouth.
In the USA it is common and accepted practice to use your aircraft type instead of "november" over the airwaves so that everybody knows what sort of performance to expect from you. If you hear "Lear1234 ten miles out" you will expect to see him long before "Cherokee4321" at a similar distance. It is simply a courtesy.