On-Off valve operates only in two modes- it is either fully open or fully closed. A tap to fill overhead tank is a good example of on-off valve. The tap is either fully open or fully closed. In case of on-off valve, there is no provision to control the flow at some intermediate level. In other words, they are not used for throttling purposes.
Control valves are modulating type of valves which can operate at several intermediate positions. The purpose of using control valve is to regulate some parameter. In case of a heat exchanger, control valves are used to control the quantity of heat transferred by modulating the flow of hot fluid.
In case of steam systems, a control valve is used to control the process temperature. Temperature and pressure of steam are directly related to each other. The control valve controls the pressure of steam thus leading to change in temperature. A temperature sensor senses the pressure temperature and sends signal to actuator which modulates the valve output to give correct output.