Essentially there are two types of shower valves, thermostatic and manual. A thermostatic valve maintains a constant set temperature. As long as the valve is set to your preferred temperature you simply turn the shower on and it will maintain that temperature. A manual valve, on the other hand, involves adjusting the temperature for your desired comfort.
One of the key differences between the valves is when someone else in the house turns on a hot tap. With a thermostatic valve you shouldn’t notice any difference to the shower temperature as the valve will compensate for the drop in hot pressure. With a manual valve the drop in hot pressure can mean that the shower suddenly becomes very cold. As a result you need to adjust the valve temperature to make the water hotter but once someone else in the house has turned the hot tap off, suddenly the shower temperature becomes too hot thus requiring more adjustment.
Most thermostatic valves require that the water pressure for hot and cold feeds is equal in order to maintain the perfect temperature.