What are the three types of heat transfer?
Conduction, convection and radiation.
Heat is energy and at the molecular level is related to the masses and the speeds at which particles of matter (electrons, atomic nuclei, atoms, molecules) are "colliding" with each other.
Conduction: heat is transferred by energy being passed from a high energy particle to its surrounding neighbors by colliding, in a similar way to billiard ball collisions.
In a solid, where particles are locked in place, unable to freely move about in the material, this is the way heat is transferred.
Convection: heat is transferred by the movement of the hotter (higher energy) particles away from the heat source, carrying the heat they have gained with them.
This can only happen in fluids (liquids and gasses) where the particles are free to move about in the space. The hotter fluid is less dense so will rise due to its greater buoyancy and cooler fluid will take its place, to be heated in turn.
In a closed environment the convection currents circulate around the space distributing the warmed particles and therefore transferring the heat.
Radiation: radiant heat energy is transferred in the form of electromagnetic radiation. Hot objects emit infra-red radiation from their surface, which then heats up any matter that absorbs it.
Radiant heat passes through infra-red transparent material (e.g. air) and warms infra-red absorbent objects that it hits. Black surfaces tend to emit and absorb infra-red radiation best.
The heat from the sun travels to Earth as radiant energy (conduction and convection are not possible in a vacuum).