Heat exchangers are systems that use a fluid to absorb heat from a hotter outside source without the fluid and hot source mixing together. Therefore, the fluid that entered hot, leaves cold and the initially cold fluid leaves hot. For example, water can be heated while inside a metal pipe within a furnace or boiler. There could ways to heat water (and cool a heat source)—like throwing water onto a fire. Although the water is now hot and fire is now cooler, they are not useful anymore since the fire is put out and the water is lost as vapour. Avoiding this mess is the essence of a heat exchanger; the fluid never has to come in direct contact with the heat source. Heat exchangers with larger surface areas are more desirable, as they allow for more thermal contact and can therefore exchange heat faster.
Power plants use heat exchangers to collect heat from hot waste gases to get power. Refrigerators use heat exchangers to dump the heat from inside the fridge to the room that it's sitting in. Vehicles use heat exchangers to dump waste heat to the atmosphere so they don't overheat. For instance, a car radiator is a type of heat exchanger. Coolant that takes heat from the engine flows through the radiator which has metal fins that open into the air. As the car drives along, air blowing through the front grille cools this coolant and the waste heat can flow into the passenger compartment, providing heat to the car.
Efficiencies of power plants can be increased with heat exchangers because the exhaust gases still have some useful energy in the form of heat. This heat is absorbed by heat exchangers in the smoke stack and brought somewhere where it can contribute usefully, such as to pre-heat the fuel going into the boiler, or to heat a nearby office.