Tornadoes are classified according to wind speed and damage. 

The Enhanced Fujita Scale
The Enhanced Fujita Scale (EF Scale) is scheduled to go into practice on Feb 1, 2007.  The scale represents better engineering studies on different types of structures and how those structures are designed.
 

EF0	Wind Speed	65-85 MPH
	Potential Damage	Light damage.   Peels surface off roofs; some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over; mobile homes pushed off foundations or overturned; sign boards damaged.
EF1	Wind Speed	86-110 MPH
	Potential Damage	Moderate damage. Roofs torn off frame houses; windows and glass doors broken; moving autos blown off roads; mobile homes demolished; boxcars overturned.
EF2	Wind Speed	111-135 MPH
	Potential Damage	Considerable damage. Roofs torn off well-constructed houses; foundations of frame homes shifted; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.
EF3	Wind Speed	136-165 MPH
	Potential Damage	Severe damage. Some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown; structures with weak foundations blown away some distance.
EF4	Wind Speed	166-200 MPH
	Potential Damage	Devastating damage. Well-constructed houses and whole frame houses completely leveled; structures with weak foundations blown away some distance; trees debarked; cars thrown and small missiles generated.
EF5	Wind Speed	> 200 MPH
	Potential Damage	Incredible damage. Strong frame houses leveled off foundations and swept away; with strongest winds, brick houses completely wiped off foundations; automobile-sized missiles fly through the air in excess of 100 m (109 yd); cars thrown and large missiles generated; incredible phenomena will occur.

The Fujita Scale (Retired on Feb 1, 2007 - No Longer Used)

(F0) Gale Tornado (40 - 72 m.p.h.) - Light damage: some damage to chimneys; break branches off trees; push over shallow-rooted trees; damage sign boards

(F1) Modeate Tornado (73 - 112 m.p.h.) - Moderate damage: the lower limit (73 m.p.h.) is the beginning of hurricane wind speed; peel surface off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off road

(F2) Significant Tornado (113 - 157 m.p.h.) - Considerable damage: Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; heavy cars lifted off ground and thrown

(F3) Severe Tornado (158 - 206 m.p.h.) - Severe damage: Roof and some walls torn off well - constructed houses; trains overturned; most trees in forests uprooted; heavy cars lifted off ground and thrown

(F4) Devastating Tornado (207-260 m.p.h.) - Devastating damage: Well - constructed houses leveled; structures blown off weak foundations; cars and other large objects thrown about

(F5) Incredible Tornado (261 - 318 m.p.h.) - Incredible damage: Strong frame houses are lifted off foundations and carried a considerable distance to disintergrate; automobile - sized missles fly through air in excess of 100 meters; trees debarked

(F6+) Inconceivable Tornado (319 - 379 m.p.h.)  These winds are very unlikely. The small area of damage they might produce would probably not be recognizable along with the mess produced by F4 and F5 wind that would surround the F6 winds. Missiles, such as cars and refrigerators would do serious secondary damage that could not be directly identified as F6 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  There is, by definition, no such thing as an F6 tornado.