First observed in 1834 by Jean Peltier, the cooling of one junction is known as the Peltier effect and occurs when a current is passed through two dissimilar semiconductors (n-type and p-type) that are connected to each other at two junctions (Peltier junctions). The current drives a transfer of heat from one junction to the other.
The conductors are attempting to return to the electron equilibrium that existed before the current was applied by absorbing energy at one connector and releasing it at the other. The individual couples can be connected in series to enhance the effect. The direction of heat transfer is controlled by the polarity of the current, reversing the polarity will change the direction of transfer.
A Peltier cooler/heater or thermoelectric heat pump is a solid-state active heat pump which transfers heat from one side of the device to the other.
See also seebeck effect, the opposite effect - applying temperature gradient to generate current.
Peltier coolers are also called TECs (Thermo Electric Converter).