Boilers are closed metal vessels, usually made out of some type of metal, in which water is heated. The heated water exits the boiler and travels through a pre-determined route (usually pipes in the walls of a building) transferring that heat to the area. Once the water gets cold it travels back to the boiler to be re-heated.
The majority of boilers heat the water passing through them by a process called combustion. This is done by lighting or burning common commodities such as wood, electricity, coal, heating oil, and natural gas. There are some that even use steam.
Waste oil boilers work the same way except for one minor difference. They oil burner in this heating system is made to run off used crankcase oils, automatic transmission fluid, hydraulic fluid, and vegetable oil (must be mixed in a 50/50 blend with the other oils because of its low flash point).
Here is a list of the three types of waste oil boilers Energy Logic provides.
Energy Logic Black Gold Waste Oil Boilers