Hurricanes and Tornadoes are measured on scales.  Tornadoes are measured on the Enhanced Fujita Scale (from EF0 to EF5) based on how much damage they produce.  Hurricanes are measured by the highest sustained winds.

The Saffir-Simpson Hurricane Scale is a 1-5 rating based on the hurricane’s present intensity. This is used to give an estimate of the potential property damage and flooding expected along the coast from a hurricane landfall. Wind speed is the determining factor in the scale, as storm surge values are highly dependent on the slope of the continental shelf in the landfall region. Note that all winds are using the U.S. 1-minute average.

 Tropical
Depression

Susatined Winds 0-38 MPH 0-62 KPH 

Tropical
Depression
#10 (2005) 

Storm Surge 0-3 ft  0-.9 m
Central Pressure  
Potential Damage Very little wind damage.  Usually, minor Flooding. However, depending on the path and strength of the storm, major flooding can occur.
Example Storms TD 10 (2005) 

Tropical
Storm
 

Susatined Winds 39-73 MPH  63-117 KPH 

Tropical Storm
Alberto (2006) 

Storm Surge 0-3 ft  0-.9 m 
Central Pressure  
Potential Damage Some light wind damage.  Usually, minor Flooding. However, depending on the path and strength of the storm, major flooding can occur.
Example Storms Alberto (2006), Allison (2001)

Category 1
Hurricane
 

Susatined Winds 74-95 MPH 119-153 KPH 

Category 1
Hurricane Ophelia (2005) 

Storm Surge 4-5 ft  1.2-1.5 m 
Central Pressure > 980 mb 
Potential Damage No real damage to building structures. Damage primarily to unanchored mobile homes, shrubbery, and trees. Also, some coastal flooding and minor pier damage. 
Example Storms Ernesto (2006), Ophelia (2005), Gaston (2004) 

Category 2
Hurricane 

Susatined Winds 96-110 MPH  154-177 KPH 

Category 2
Hurricane Erin (1995)

Storm Surge 6-8 ft.  1.8-2.4 m 
Central Pressure 965-979 mb 
Potential Damage Some roofing material, door, and window damage. Considerable damage to vegetation, mobile homes, etc. Flooding damages piers and small craft in unprotected anchorages may break their moorings 
Example Storms Juan (2003), Erin (1995), Floyd (1998) at landfall 

Category 3
Major
Hurricane 

Susatined Winds 111-130 MPH 179-209 KPH 

 
Category 3
Major Hurricane
Fran (1997)

Storm Surge 9-12 ft.  2.7-3.7 m 
Central Pressure 945-965 mb 
Potential Damage Some structural damage to small residences and utility buildings, with a minor amount of curtainwall failures. Mobile homes are destroyed. Flooding near the coast destroys smaller structures with larger structures damaged by floating debris. Terrain may be flooded well inland. 
Example Storms Fran (1997), Katrina (2005) at landfall in Louisiana 

 Category 4
Major
Hurricane

Susatined Winds 131-155 MPH  210-249 KPH 

 
Category 4
Major Hurricane Hugo (1989)

Storm Surge 13-18 ft.  4.0-5.5 m 
Central Pressure 920-944 mb 
Potential Damage More extensive curtainwall failures with some complete roof structure failure on small residences. Major erosion of beach areas. Terrain may be flooded well inland. 
Example Storms Hazel (1954), Charley (2004), Hugo (1989) at landfall in Charleston, SC 

Category 5
Major
Hurricane
 

Susatined Winds 156+ MPH  250+ KPH 

Category 5
Major Hurricane
Isabel (2003) 

Storm Surge 19 ft.+  5.5 m + 
Central Pressure < 920 mb 
Potential Damage Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. Flooding causes major damage to lower floors of all structures near the shoreline. Massive evacuation of residential areas may be required. 
Example Storms Camille (1968), Andrew (1992), Isabel (2003), Katrina (2005), Rita (2005), Wilma (2005) 

Category 6
 

Susatined Winds Unknown Unknown 

There is no category 6

Storm Surge Unknown Unknown 
Central Pressure Unknown
Potential Damage Inconceivable.  While there have been proposals to add a “Category 6″ with winds over 175-180 MPH, none have been successful. 
Example Storms None.  Sample candidates for category 6 storms would include Wilma (2005), Allen (1980), Camille (1968) 

Other places in the world use scales that may have different measurements and terms.  You may hear of “Cyclones” and “Super Typhoons”.  In the Atlantic (and Eastern Pacific), these would be called “Hurricanes”.  You can learn more about the scales and terms used around the world on the web