The transfer of heat by the movement of a fluid such as air or water is called convection.
Convection occurs because a moving fluid carries thermal energy from one place to another. In natural convection, heating changes a fluid's density: warmer, less-dense fluid rises while cooler, denser fluid sinks. This circulation forms convection currents, such as those in a pot of boiling water or in warm air rising above a radiator.
Forced convection uses an external device, such as a fan, pump, or stirrer, to move the fluid. A car's cooling system, a refrigerator fan, and heated air circulated through a building are familiar examples. In practice, convection also includes some conduction within the fluid, but the bulk movement is the defining feature.
A common mix-up is treating convection as identical to conduction or radiation. Conduction transfers energy through microscopic interactions without bulk flow, while radiation travels through electromagnetic waves and needs no fluid or material medium.