Conduction is like a relay race, with molecules passing on their energy by jostling their neighbours. Radiation is the heat rays you feel from an open fire. Heat rays are called infrared. Like light, they are electromagnetic and can pass through empty space. Convection Heat makes molecules move further apart, so substances expand as they get hotter.
Warm air is less dense than cold air and rises.
A candle flame, like all flames, radiates heat in all directions in the form of infra-red rays. The Sun's heat reaches the Earth in the same way, travelling through space at the speed of light. The heat of the flame also creates convection, drawing in cold air, warming it and making it rise. Heat and temperature Heat is molecules moving. The faster they move, the hotter it is.