Diesel engines, unlike petrol engines which rely on a spark to ignite the air-fuel mixture, function due to the auto-ignition of diesel fuel when subjected to high pressures. Under high pressure, the temperature of the diesel fuel increases to a point where the fuel self-combusts. In cold weather however, this temperature is not always achieved and glow plugs assist by heating the air-fuel mixture to assist in cold starting.
Put simply, glow plugs are small heaters.
NGK Part Numbering System.