When you ask your thermostat for heat, your boiler begins using oil, gas, or electricity to heat up the water inside it.
Once the water/steam is hot enough, it is sent to the rest of the house to the radiators or radiant heat through the connected pipes.
The heat is transferred from the water into the surrounding air, the cold water now returns to the boiler to be reheated.
The process continues until your room is at the desired temperature.
A lot of homes still use boilers. They provide even heat, they typically last longer than furnaces, and they don’t kick up dust and other airborne irritants into your rooms.