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