A tornado can range from a relatively small event to a system capable of incredible destruction(although rare on our shores). The Fujita scale is the official measurement of a twister.

F-Scale Number Intensity Phrase Wind Speed Damage
F0 Gale
Tornado 64-115 km/h
Damages chimneys;
Breaks branches off trees;
Pushes over shallow-rooted trees;
Damages sign boards.

F1 Moderate Tornado 116-179 km/h
Peels surface off roofs;
Pushes mobile homes off foundations or overturns them;
Pushes moving autos off the roads;
May destroy attached garages.

F2 Significant Tornado  180-251 km/h
Considerable damage.
Tears roofs off frame houses;
Demolishes mobile homes;
Pushes boxcars over;
Snaps or uproots large trees;
Light objects become missiles.

F3 Severe Tornado 252-330 km/h
Tears of roof and some walls off well-constructed houses;
Overturns trains;
Uproots most trees in forest.

F4 Devastating Tornado 331-416 km/h
Levels well-constructed houses;
Blows structures with weak foundations off some distance;
Throws cars;
Large objects become missiles.

F5 Incredible Tornado 417-509 km/h
Lifts strong frame houses off foundations and carrys them considerable distances to disintegrate;
Tosses automobile-sized objects through the air in excess of 100 meters;
Debarks trees;
Badly damages steel reinforced concrete structures.

F6 Inconceivable Tornado 510-606 km/h
Such winds are highly unlikely.
The small area over which the damage they might produce would probably not be recognizable within the mess produced by F4 and F5 winds 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.