F-0 and F-1 tornadoes are considered "weak," F-2 and F-3 are "strong" and F-4 and F-5 are "violent."

F-6 or higher rated tornadoes aren't thought to exist. The damage they would do would be "inconceivable." The National Weather Service accepted the Fujita Scale for use in 1973 and Allen Pearson, then director of the National Severe Storms Forecast Center, added the Pearson Scales for tornado path length and path width, creating the Fujita-Pearson Scale. The scale had been calculated through F-12, which is Mach 1 - the speed of sound (750 mph) - but tornado wind speeds are not expected to reach these speeds; see the F-6 description below.

F-0 Gale tornado (40-72 mph): Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages sign boards.

F-1 Moderate tornado (73-112 mph): The lower limit is the beginning of hurricane wind speed; peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off the roads; attached garages may be destroyed

F-2 Significant tornado (113-157 mph): Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; light object missiles generated.

F-3 Severe tornado (158-206 mph): Roof and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted.

F-4 Devastating tornado (207-260 mph): Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated.

F-5 Incredible tornado (261-318 mph): Strong frame houses lifted off foundations and carried considerable distances to disintegrate; automobile sized missiles fly through the air in excess of 100 meters; trees debarked; steel-reinforced concrete structures badly damaged.

F-6 Inconceivable tornado (319-379 mph): These winds are very unlikely. The small area of damage they might produce would probably not be recognizable along with the mess produced by F-4 and F-5 wind that would surround the F-6 winds. Missiles, such as cars and refrigerators would do serious secondary damage that could not be directly identified as F-6 damage. If this level is ever achieved, evidence for it might only be found in some manner of ground swirl pattern, for it may never be identifiable through engineering studies. F-0 and F-1 tornadoes are considered "weak," F-2 and F-3 are "strong" and F-4 and F-5 are "violent." The Fujita-Pearson Scale adds categories to include path length and path width.

http://www.usatoday.com/weather/wfujidef.htm
http://www.fema.gov/library/tornadof.htm