Think heat outlets.
Where that magic heat from the heat source finally radiates out into your room. All radiators must be designed according to flow temperatures. For example, a lower temperature will require more exchange surface to transfer heat energy effectively. This is achieved with densely stacked piping and/or chambers for the water carried heat to radiate into the air of your home.
Radiators have a myriad of forms, all built to meet needs of function and aesthetic design.