The big news right now is the near-complete destruction of Greensburg, Kansas. Elsewhere in the southern Great Plains another outbreak is occurring. While nowhere near the destruction Catholicgauze is receiving a lightning show with periods of downpour.
The basic science and geography of tornadoes is excellently explained over at Geography@About.com's Tornado page. Be sure to also check out information on the Fujita Scale.
In the United States most Tornadoes occur between the Rocky Mountains and Appalachian Mountains. The greatest concentration is in Tornado Alley (be sure to check out the maps over at Wikipedia's page for Tornado Alley and Tornadoes). While there is no official Tornado Alley, the Great Plains stretch from northern Texas to South Dakota is a good rule of thumb. Internationally Bangladesh, Europe, South Africa, Australia, and China are known for their moderate to severe tornadoes.
If one is in the area of severe weather with the possibility of tornadoes be sure to seek shelter in a basement or an interior room away from windows. If one is outside seek safety in a ditch. NEVER seek safety under an overpass. These areas will become wind tunnels and one risks being sucked in by sever gusts.
The U.S. Severe Weather Map at Weather Underground maps out all areas at risk of severe storms and tornadoes.