A thermoelectric cooler (TEC) is a small cooling device that relies on a Peltier junction. Composed of two conductors made of different materials, a Peltier junction (discovered in 1833 by J.C. Peltier) acts as a heat pump which can cool or warm when current is passed through it.
The small size of the TEC allows precision thermal control of individual components such as fiber optic laser drivers, precision voltage references, or any other temperature critical device. Temperature-critical components are integrated with a TEC and a temperature monitor into a single thermally-engineered module.
A "thermoelectric controller" (also abbreviated TEC) is an electronic circuit that controls the current that drives the junction. These can be quite sophisticated. Many can drive a positive or negative current (so they can heat or cool), use PWM for efficiency, and incorporate control to regulate the amount of current. Examples of such circuits are linked below.
See: App Note 3318, HFAN-08.2.0: Thermoelectric Cooler (TEC) Control.
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