All of these gas appliances are categorized by their design.

B-vent:

B vent (or naturally aspirated) must exhaust vertically and may also have a formal combustion air vent connected from the fireplace to the outside air, or use the room air for combustion.

This is technology that was used very commonly in the early days of gas fireplaces design.

Direct vent:

Direct vent appliances are also sometimes referred to as sealed combustion systems.

This type of appliance may exhaust vertically or horizontally but feature more than one vent (one pipe for exhaust and one for combustion air to burn)

Power vent:

Direct vent appliances are sometimes used in conjunction with power vent systems.

This type of appliance may exhaust vertically or horizontally but feature more than one vent (one pipe for exhaust and one for combustion air to burn) and will have a power vent either externally, in line, or internally, that will not allow the fireplace to operate unless a draft has been established.

Vent free:

Unlike direct vent or B vent models, all vent free appliances have no venting for exhaust or fresh air intake.

All vent free appliances operate in the same way and use room air for combustion and exhaust directly back into the room. All vent free appliances feature a unique component known as an O.D.S. (oxygen depletion sensor) pilot system to monitor the level of oxygen within the room.

If the oxygen level within the room drops below a certain level (18%), The pilot will cease to function and not allow the burner(s) to fire or gas to flow.

ODS sensor:

Outdoor (ventless):

These may or may not be vented but are all designed for outdoor spaces or covered outdoor spaces with the minimum required openings for adequate ventilation.

These may look similar to vent free fireplaces and operate in the same manner with no venting, but do not have an O.D.S. (oxygen depletion sensor) and are usually designed for exposure to the elements.