One can google on "heat exchangers". Is this a homework problem?
The main purpose of a heat exchanger (HX) is to "exchange heat", most commonly from one working fluid to another, while maintaining separation of the two fluids, which could be the same fluid, but under very different thermophysical conditions, or two distinctly (chemically) different fluids.
The principal elements of the HX are the shell (vessel) containing one fluid, the tubes which contain and transport the other fluid, and support structures (including baffles).
Any industry that uses heat will like have some kind of heat exchanger(s) involved in whatever process uses the heat.