heat engine
Every car engine, steam turbine, and power-plant boiler does the same basic trick: it sits between something hot and something cold and skims off useful motion as heat tumbles from the one to the other. A heat engine is any device that turns a flow of heat from a hot place to a cold place into work — the spin of a shaft, the push of a piston.
The arrangement always has three parts: a hot reservoir that supplies heat, a cold reservoir that receives the leftover, and a working substance (steam, fuel-air gas, a refrigerant) that cycles between them carrying energy. Crucially, the engine cannot turn all the incoming heat into work; the second law forces it to dump a portion to the cold side just to complete each cycle.
Why it matters: heat engines are how civilization converts fuel and sunlight into motion and electricity, so their limits set the limits of much of our technology. The unbreakable caveat is that some heat must always be wasted — an engine that turned heat fully into work, drawing on a single temperature, is forbidden by the second law (a 'perpetual-motion machine of the second kind'). The waste is not bad engineering; it is the price of the arrow of time.
A coal power plant boils water with burning coal (the hot reservoir, near 800 K), spins a turbine with the steam, and condenses the spent steam against a river or cooling tower (the cold reservoir, near 300 K). Roughly a third of the coal's energy becomes electricity; the rest leaves as warm water and exhaust — wasted by law, not by sloppiness.
Hot in, work out, waste heat to the cold side — the universal shape of a heat engine.
Run a heat engine backward and you get a refrigerator or heat pump: input work to push heat from cold to hot. Same machinery, opposite direction — and still bound by the second law.