Radiators work by passing their heat to the surrounding air. But they don't just radiate heat in fact most of the heat passed off a radiator is in the form of convection. Some radiators come with fins at the back, these are there to create a bigger surface area.
Two common faults:
If the radiator is cool at the top whilst still hot at the bottom it will need to be bled.
And if it is the other way round where its the top thats hot and the bottom thats cool then the radiator contains sludge.