What is a tornado?

A tornado is a violent, whirling windstorm that crosses land in a narrow path. It can result from wind shear and updrafts inside a thunderstorm. The usual form is a funnel-shaped cloud with destructive spinning winds, that descends from a cloud to the ground over a relatively short period of time.

Powerful tornadoes create the highest wind speeds on the surface of the Earth, which may be in excess of 200 mph (320 kph). The winds and debris will destroy houses and similar structures, and may cause numerous fatalities. Tornadoes are spawned by mesocyclones, and also form as tropical cyclones come ashore.
Tornadoes are classified using the Enhanced Fujita damage scale, from EF-0 (weakest) to EF-5 (strongest). The vast majority of tornadoes are comparatively small and weak storms that last only a few minutes to an hour. Less harmful tornadoes that form over water are called waterspouts.

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According to the Glossary of Meteorology (AMS 2000), a tornado is "A violently rotating column of air, pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud." Literally, in order for a vortex to be classified as a Tornado, it must be in contact with the ground and the cloud base. Weather scientists haven't found it so simple in practice, however, to classify and define Tornadoes. For example, the difference is unclear between an strong mesocyclone (parent thunderstorm circulation) on the ground, and a large, weak tornado. There is also disagreement as to whether separate touchdowns of the same funnel constitute separate tornadoes. It is well-known that a tornado may not have a visible funnel. Also, at what wind speed of the cloud-to-ground vortex does a tornado begin? How close must two or more different tornadic circulations become to qualify as a one multiple-vortex tornado, instead of separate tornadoes? There are no firm answers.

(Tornado FAQ - see related link )