What is the difference between heat exchanger and heater?

A heat exchanger is a device that transfers heat between two or more fluids without mixing them, allowing for efficient thermal energy exchange in processes like heating, cooling, or energy recovery. In contrast, a heater is specifically designed to increase the temperature of a single fluid, typically by adding energy to it, such as in electric or gas heaters. While both devices involve heat transfer, their functions and applications differ fundamentally.