Tornado wind speeds can reach 300 mph, and most tornadoes move at about 35 mph. The biggest and most dangerous tornadoes result from rotating thunderstorms called supercells. They also cause flash floods and hail.
Tornadoes are classified using the Fujita scale. This ranges from an EF0, which is the weakest tornado, to an EF5, which has a rotational speed of at least 200 mph. This class of tornado debarks trees, levels buildings and throws debris over 100 yards.
According to statistics from CNN, in late April 2011, 201 tornadoes across six states caused 321 fatalities. Most of the deaths were in Alabama. The deadliest, single tornado occurred in Joplin, Mo. in May 2011, killing approximately 158 residents.
About.com explains that tornadoes are most likely to occur in an area nicknamed Tornado Alley. They most frequently occur in the United States, but are also seen in Japan, Great Britain and other countries. Tornadoes are more likely in the late winter and early spring, but can happen in any season.