Belleville washers are also known as cupped spring washers. Its shape is conical and for that reason this type of washers have a spring characteristic. Although some people use Belleville washer to fix appliances like stoves cookers and refrigerators as well as washing machines, they are mostly seen in formula one cars, due to their tuning ability. They are also widely used as a locking mechanism. If you stack them you can use them to change the spring constant, pretty cool, right?
Belleville washers come in several colours. Bronze, silver, red, yellow, black, and many more. Belleville washers are not easy to use. You can find some information on how to use them in some websites. So do your research.