Boilers come in two varieties of steam and hot water. A boiler either heats up water or creates steam that travels through pipes in your home to radiators in older homes or baseboards in newer homes. Water is then returned back to the boiler once the steam cools or the hot water has traveled through the pipes. It is then heated up again and the process repeats.
Radiators are usually found in older homes and have been for the most part phased out by more efficient baseboard. Radiators resemble metal tubing bunched together in usually tall and wide constructs. They can be covered to blend in with your home decor as well. Most radiators look similar to the image below.
Baseboards are radiators that are spread out along the floor of the room that is heated. They allow for the heat to evenly heat a room instead of being centralized in one area like a radiator. Baseboards look similar to the one pictured below.