A Boiler is a gas unit device to boil the water to heat the house through floor radiant heat or baseboard radiant heat. Each boiler at least consists of a circulation pump, zone valve, shut off valve, expansion tank, drain valve with a supply hot water line and return water line to heat up again and send back to the house.
There are few types of boilers, High efficiency and mid efficiency. The high efficiency has combi system for heat the house and boil the domestic water heater (DWH), and the unit heat only.
Also with a boiler you can have extra DWH through a indirect tank, this give you as much as you need hot water and never you were short of hot water, depend on the size of indirect tank. Boiler is used for heating small houses that are huge where a furnace would not handled that big size. All the high rises, big commercial spot and all industrial places such as green house, factories all will be heat up with a boiler/boilers.