The three modes of heat transfer are conduction, convection, and radiation. All three mechanisms involve the transfer of thermal energy from a warmer object to a cooler object.
Conduction is the transfer of heat through direct contact between particles. When two objects are in contact with each other, heat will flow from the warmer object to the cooler object until both objects reach the same temperature. The rate of heat transfer depends on the material properties of the conductor (such as thermal conductivity), as well as the magnitude of the temperature difference between the two objects.
Convection is the transfer of heat via fluid motion. When a fluid (liquid or gas) is heated, it expands and becomes less dense than its surroundings. This causes it to rise upwards while cooler, more dense fluid flows in to take its place. The result is a large-scale circulation pattern known as convection currents. Convection can occur naturally (e.g., wind) or be induced by mechanical means (e.g., fans).
Radiation is the transfer of heat via electromagnetic waves. Unlike conduction and convection, which require matter to be in physical contact with each other, radiation can occur through a vacuum—making it an important mechanism for heat transfer in space! Radiation transfers energy by absorbing and emitting electromagnetic waves; these waves carry energy away from their source (in this case, a hot object). The amount of radiation emitted by an object depends on its surface area and temperature; hotter objects emit more radiation than cooler ones.