A wood burning fireplace is either fueled by a manufactured product (pellets or petroleum based artificial logs) or wood. The efficiency is no better than 33% and the by-products are unknown. This segment of the market is shrinking rapidly.
Direct Vent Gas Fireplaces:
Coaxial vent system of a pipe inside a pipe (generally 4"/7") the inside pipe is for exhaust, while the outer pipe brings in outside air for combustion. The design of these fireplaces allow for a completely sealed system that does not compromise inside air quality of your home. It also lets you vent a product horizontally (great for when an appliance is installed on an outside wall). Direct vent appliances are usually limited to a length of 40ft (with a maximum horizontal length of 20ft) that will allow an appliance to function properly.
Vented Gas Fireplaces
Uses air-cooled pipe (B-vent), or gas flex liners to vent appliances. These types of appliances use room air for combustion and exhaust through the B-vent pipe or flexible gas liner. Vented appliances are not as restricted in venting length as compared to direct-vent appliances and can utilize existing chimney systems by running a gas flex liner into existing chimneys and terminating with a raincap and flashing.
Vent Free Gas Fireplaces
(For use in the USA only, check local jurisdictions)
No chimney systems are required and this product can be used almost anywhere in your home (except in bathroom, bedroom, spa room, mobile home, or recreational vehicle), but provisions for adequate combustion and ventilation must be provided (room size and btu's of appliance are the deciding factors of what btu size can be used for a particular installation). The appliance uses room air for combustion and exhausts to the room. These appliances are 99.9% efficient and the vent-free burner systems have an ODS pilot system, which ensures that the burner systems will shut down if oxygen in the room falls below acceptable limits.