In a direct-fired unit all the heat and all the combustion gas end up in the room. Efficiency is nearly complete, but the space must be ventilated and water vapour is released.
An indirect-fired unit has a heat exchanger and a flue: the exhaust leaves the room and only clean warm air stays behind. Required for food storage, marquees with an audience and drying work.
Infrared radiant heaters warm surfaces and people rather than air. No fan means no dust and no draught, ideal for terraces, work stations and semi-open spaces.
Electric units have no such distinction: there is no combustion, but the available power on the connection limits the choice.