While wood boilers typically function as central boilers, they're typically placed outdoors, while other central boilers are generally placed inside. Most wood boilers are significantly less efficient than central boilers, but people using them in forested areas can effectively eliminate their heating bills.
Natural gas, propane and heating oil burn fairly cleanly, and their combustion by-products are easy to vent. Wood, on the other hand, produces far more dangerous chemicals and particulate matter. In addition, different types of wood have different properties, so most boilers and furnaces are placed outdoors.
Wood has a few advantages as a fuel source. Homeowners with a sufficient number of trees can avoid paying for oil or natural gas. If trees cut down are replaced, the new trees absorb carbon dioxide, so many consider wood to be a "carbon neutral" fuel. Wood boilers are also fairly simple to operate and easy to service, so people in remote areas can often fix problems without needing expert help.
Outdoor wood boilers are often regulated differently than indoor gas and oil boilers. Since wood boilers release so much particulate matter, some areas require installing more efficient units and may even ban the installation of new units. In remote areas where natural gas connections are not available and oil is expensive to transport, however, wood boilers may be a sound investment.