Direct injection diesels inject the diesel fuel directly onto the piston crown. Most direct injection diesels have a swirl chamber designed into the piston. Due to the high compression required for a direct injection diesel engine to operate diesel noise is usually more pronounce. The higher compression creates more compression heat so the use of glow plugs is not required for cold starts.
Indirect injection diesel engines inject the diesel fuel into a pre- combustion chamber. This chamber has a glow plug that aids the compression heat to assist with cold starts. Indirect injection diesels usually have a lower compression ratio and are much quieter running.