When you classify tornadoes the main ingredient that classifies the tornado is the wind speed.  By measuring the wind speed you can tell whether or not the tornado is weak or very devasting.  A weak tornado(F1) has wind speeds between 73-112, while devasting tornado(F5) has wind speeds between 207-260.
 
 

F1 TORNADO: WEAK
 

Tornadoes are the weakest tornadoes, with winds of approximately 73-112 miles per hour.  They tip over mobile homes and cars and down trees and power lines.  Their damage is minimal, however, and compare to straight-line winds of similar strength.  During their life span, many tornadoes start as an F1, become stronger, and then end again as an F1.  Tornadoes in the "rope" stage of development and waterspouts (tornadoes located over a body of water) are generally regarded as F1 in variety.

F2 TORNADO: STRONG

Tornadoes have winds of approximately 113-157 miles per hour, and cause more significant destruction than their weaker counterparts.  Roofs are torn from homes, weaker structures are destroyed, trees are uprooted, and cars are blown of highways.  Shelter must be taken to protect lives from these tornadoes.

F3 TORNADO: SEVERE

F3 tornadoes have winds of approximately 158-206 miles per hour.  They are extremely destructive, having the potential to be vicious killers of anyone in their path.  Roofs and some walls torn off well-constructed fame houses; some rural buildings completely demolished or flattened; trains overturned; steel framed hangar-warehouse type structures torn; cars lifted off ground and may roll some distance; most trees in a forest uprooted, snapped, or leveled; block structures often leveled.  Shelter must be taken to survive.

F4 TORNADO: DEVASTATING
 

F4 tornadoes are noted for winds of approximately 207-260 miles per hour.  Well constructed frame houses leveled, leaving piles of debris; structures with weak foundation lifted, torn, and blown off some distance; tree debarked by small flying debris; sandy soil eroded and gravels fly in high winds; cars twisted and thrown some distances or rolled considerable distances.  Shelter must be taken outside of automobiles and beneath ground if possible.

F5 TORNADO: INCREDIBLE

F5 tornadoes are the most violent and destructive types of tornadoes.  They usually have very large bases, with wind speeds of around 261-318 miles per hour.  These tornadoes result in complete destruction and annihilation of virtually anything in their path.  Strong frame houses lifted clear off foundation and carried considerable distance; steel reinforced concrete structures badly damaged; automobile-size missiles fly through the distance of 100 yards of more; trees debarked completely.  They have the ability to strip off the pavement from roads, make deadly missiles out of ordinary objects and completely obliterate buildings and homes.  F5 tornadoes are not survivable unless their victims are safely underground or otherwise sufficiently protected from their winds.

WATERSPOUTS

 They are funnel clouds like tornadoes, except they occur over water.  They are common in all equatorial oceans and inland seas.  High surface temperatures and humidities are necessary for their formation.  Waterspouts are similar to tornadoes in that they are a part of a cloud system.  The water in the waterspout is from the condensation of water vapor in the air that is being pulled into the updraft within the cloud.  Waterspouts can occur in good weather with only small clouds present.  They are rather storms and rarely cause much damage.
LANDSPOUTS

They are just like waterspouts in that they are not from the fierce thunderstorms known as supercells(that produce the most violent tornadoes).  Like waterspouts, landspouts form from cumulus congestus clouds, except instead of being over water they are over land.  These clouds often aren't associated with thunder or lighting and although they may be producing some rain the storm is not violent.  The resulting funnel cloud, the landspout, is not violent either.  Landspouts are weak and generally do not produce much damage.

DUST-DEVILS

They're smaller in size than tornadoes, but they can be very powerful.  Dust-devils are commonly seen in the American southwest and often cause major damage.  These storms are not connected to clouds like the regular tornado.  The energy for the storm originates from the hot, dry ground interacting with an isolated pocket of rolling air above it, creating a whirlwind cloud of dust.  Dust-devil speeds can exceed 100 km/h.