Many homes in South Orange and Maplewood, NJ use steam heat, an outdated but very comfortable heat source is simple to understand, but requires routine attention for proper operation.
Steam boilers (not furnaces as they are sometimes called) can be heated by oil or gas which are called having an oil fired steam boiler or a gas fired steam boiler.
Think of a boiler like a pot of water on your stove with a flame under it. In a boiler, as the water heats up it turns to steam which rises into the large steam pipes in the basement, flows up to the radiators where the steam will condense releasing heat in the radiator. The condensate water will than flow back to the boiler in the same pipes it entered and through the condensate return pipe located in the basement. Keep in mind that the steam system piping are installed at a set height and can not be relocated. If they are hanging low and a head banger you have to live with it.
The steam will only enter the radiators if the air vent valve located on the opposite side of the steam intake valve is working properly. If the air vent does not open the radiator will not heat up. If the vent valve is broken it will allow steam to be release into the rooms. The size of the air vent regulates how fast each individual radiator will heat up. Remember a steam system can only be controlled by one thermostat usually located in a central first floor room. So installing the correct size air vent is important to have a balanced system for even heat distribution to each room.
If the steam piping in the basement or the radiators are no longer sloped properly (due to settlement of a home) to allow the water to flow back to the boiler by gravity, a loud hamming noise will be heard. This is corrected by re-leveling the steam piping and radiators. Wood house structures shrink and settle over time where as the metal steam piping does not.
Now about maintenance: Water seeks its own level so the water you see in the sight glass on the side of the boiler is the level of the water in the boiler. The water level must be maintained between 1/4 to 3/4 in the sight glass. If the water is too low the water cut off switch connected to the boiler will shut down the boiler to stop the boiler from cracking like what would happen if you where heating an empty pot on the stove. Just add water to restart the boiler.
The cleaner the water in the boiler, the lower the boiling point, so flushing out the boiler at the beginning of the season and draining the boiler and adding clean water periodically during the winter will make the boiler run more efficient.
Hope this helps in understanding your boiler.