The ICAO8643 code is also known as the aircraft type code or, in Flight Simulator X, the aircraft model code. It is a short alphanumeric code assigned by ICAO that indicates the manufacturer and model of the aircraft. For example, B744 is the ICAO8643 code for Boeing 747-400s while B738 is the code for Boeing 737-800s.
The ICAO8643 code can usually be used to infer the species of aircraft (landplane, seaplane, helicopter etc.) and the type and number of engines. However this isn't an exact science because sometimes the same code is allocated to entirely different models of aircraft, sometimes even from different manufacturers.
A full list of ICAO8643 codes can be found at http://www.icao.int/publications/DOC8643/Pages/default.aspx.