Gary - Generally, diesel engines come in two groups, based on the type of fuel system / combustion that they have:
1. Direct Injection (DI): High pressure fuel is directly injected into the combustion chamber. No glow plugs are required. This is the most efficient method, and the most common on today's larger (50hp & up) engines. It does require a more expensive injection pump to develop the pressure & accurately deliver the fuel. It also requires good quality & clean fuel (easy in NA, but not worldwide). Due to speed of injection & the lack of glow plugs, these types of engines will often produce white smoke (unburned fuel) during a cold startup.
2. Precombustion (PC): Medium pressure fuel is injected into a precombustion chamber, where the heat is initially generated, then the fire, fuel & expanding gasses travel out to the main combustion chamber (where they complete their burn as they mix with additional fresh air). This combustion type generally requires glow plugs. It is also slightly less efficient, but can burn any fuel you can get your hands on.
The above two types are general rules, and there are many examples around the world where many manufacturers have tried variations of the above two groups. Interestingly, in an effort to improve the efficiency of gasoline-powered engines, several manufacturers are testing DI-type systems (today's fuel injected gas engines are very similar in concept to the PC system, other than that the fire is not started until the spark is generated).