AS of 2014, most self-cleaning ovens work either by heating the oven to around 1,000 degrees Fahrenheit or by using a combination of lower temperatures and steam. Both methods are effective at loosening up any spills or leftover food particles from the oven's interior, but they do so in different ways.
In the ovens that use extreme heat, the temperatures turn to ash anything left inside the oven. The ash is then easily wiped away when the cleaning process is over and the oven has cooled. The steam method works in much the same way except that the steam merely loosens any particles that are stuck to the oven's surface so that they can also be wiped away later.
Both methods are effective largely due to the special enamel coating used on the ovens' interior surfaces. This coating makes the surfaces smooth and helps to ensure everything is wiped off easily once the cleaning is finished.
There is also a third type of self-cleaning oven known as a continuous cleaning oven, but these are very rarely if ever produced as of 2014. In these ovens, the enamel has a more porous texture designed to spread spills out over a larger surface area to make them easier to clean up without the need for excess heat.