All portable heaters have different energy supplies which are then converted to heat in different ways.

Basically there are two different types of oil/gas heaters and these are i) Direct heaters and ii) Indriect heaters. Then there are also electric heaters:

Direct Heaters

In a direct fired system (gas heater or simple oil heater) the fan which blows out the heated air to the room also supplies the heater with the combustion air. The fuel is atomised in the combustion chamber, ignited and burnt. The hot gases are subsequently mixed in with the air form the fan.

For this reason this process has a 100% output, since the entire amount of heat generated is available to the room. 
However, direct heaters also release exhaust gases and moisture from the combustion process directly into the room, therefore these rooms must be very well ventilated.

Expert Tip: Direct heaters are very unsuitable for building drying, because for every litre of fuel burnt approximately 1.64kg of additional water vapour is generated which condenses on the buildings surfaces.

Indirect Heaters

Gas or oil fired/fuelled heaters, however, with an indirect heater the heated air being supplied to the room is completely separate from the combustion chamber of the heater. Indirect heaters generate clean, dry heat without combustion gases or water vapour entering the room.

Electrical Heaters

These heaters offer the safest, fastest and most convenient method of heating a relatively small area. Electrical heaters are distinguished by the methods of transporting the heat. In the case of fan heaters, the heat which is generated is transported via an internal fan and distributed evenly throughout the room. In the case of infra red heaters, the heat is transported by light waves.

The best heater to use always depends on the respective application. For example never use a direct heater where there is a risk of fire due to the presence of flammable material. Always use an indirect heater or electrical heater where there is limited ventilation. Electrical heaters should be used to heat interior rooms or when you want to dry out a building faster!