A boiler room can be found into residential apartement complexes, schools and hospitals, but will almost always be present in industrial sites.
A boiler room consists of a big boiler tank in which water is heated by natural gas, fuel oil, or rarely these days: coal. The steam produced is of high, medium or low pressure. The steam can be used to heat the building, produce movement in machinery, furnish heat to industrial equipment etc.
Steam is way more hot than boiling water and travel faster in pipes. Steam pipes are always insulated, in old industrial sites those pipes could be insulated with asbestos. Steam tunnels always begin at a boiler room and end up at the delivery building.