A tornado is rated by its wind speed. The scale for rating the wind speed is called the Fujita tornado scale  (click to see scale). They are ranked from F-0 to F-6, but an F-6 has never occurred. A F-0 tornado will cause damage to small trees, but an F-5 will destroy almost everything in its path.

How does a tornado work? Take a look at the picture below and try to follow along to what I write. Simply, a tornado is a vortex of wind energy that is created by the updrafts of a super cell tornado which, along with the coriolus effect (the cause of your toilette to swirl the water counter clockwise) spins around. At a point, like in the middle of a whirlpool or  a HURRICANE it spins faster and faster, where we call it a tornado. As you can see in the picture, the tornado is actually just a small part of the whole updraft of the cell. However, since it is as powerful as it is, the purpose behind it is still to bring more moisture and energy to the cell. The tornado is very similar to a HURRICANE only on a much smaller scale.
 

Though a tornado is strong, you should have an idea of what its rating is all about so you know what to expect when one hits. There is a humorous rating system of tornadoes located on Intellicast which you may be able to better relate to the intensity of these vortices. The above links and those on my LINKS page should be good sources for your information. Good luck, and again, should you have any questions still, E-MAIL me!