In order to control the flow of coolant to the engine, a thermostat is used. This mechanical component opens and closes based on temperature, which allows the engine to run at a given temperature.
Pictured at the left is a new 44mm thermostat, which is designed to open at 195°F/90°C. This Stant Superstat was produced in the USA.
A bad thermostat can cause a number of problems in the cooling system. If it is stuck closed, the lack of coolant flowing through will cause the engine to overheat. If it is stuck open, the engine may take a long time to reach optimal operating temperatures, which may cause fuel injected models to consume more fuel. If the thermostat still functions, but is worn out/gummed up, you may notice your vehicle's temperature guage getting really high and low (sometimes quite quickly) since it can't react as quickly as the temperature changes.