The Saffir-Simpson Hurricane Scale
Category
Category 1 Storm
Sustained Winds: 74 - 95 mph 119 - 153 km/h
Central Pressure: 28.94 inHg 980 mbar
Potential Damage: No real damage to building structures. Damage primarily to unanchored mobile homes, shrubbery, and trees. Some damage to poorly constructed signs. Also, some coastal road flooding and minor pier damage.
Examples: Hurricane Lili (2002) - landfall on the Louisiana coast.
Hurricane Gaston (2004) - landfall along the central South Carolina coast.
Category
Category 2 Storm
Sustained Winds: 96 - 110 mph 154 - 177 km/h
Central Pressure: 28.50 - 28.91 inHg 965 - 979 mbar
Potential Damage: Some roofing material, door, and window damage to buildings. Considerable damage to shrubbery and trees. Considerable damage to mobile homes, poorly constructed signs, and piers. Coastal and low-lying floods. Small craft in unprotected anchorages break moorings.
Examples: Hurricane Frances (2004) - landfall over the southern end of Hutchinson Island, Florida.
Hurricane Isabel
(2003) - landfall on the Outer Banks of North Carolina.
Category
Category 3 Storm
Sustained Winds: 111 - 130 mph 178 - 209 km/h
Estimated Storm Surge: 27.91 - 28.47 inHg 945 - 964 mbar
Potential Damage: Some structural damage to small residences and utility buildings. Damage to shrubbery and trees with foliage blown off trees and large trees blown down. Mobile homes and poorly constructed signs are destroyed. Some flooding in low-lying areas. Flooding near the coast destroys small structures with larg structures damaged by battering from floating debris.
Examples: Hurricanes Jeanne and Ivan (2004) were Category Three hurricanes when they made landfall in Florida and in Alabama, respectively.
Category
Category 4 Storm
Sustained Winds: 131 - 155 mph 210 - 249 km/h
Estimated Storm Surge: 27.17 - 28.88 inHg 920 - 944 mbar
Potential Damage: Some complete roof failure and structure failures on small residences. Shrubs, trees, and all signs are blown down. Complete destruction of mobile homes. Extensive damage to doors and windows. Low-lying flooding. Major damage to lower floors of structures near the shore.
Examples:

Hurricane Charley (2004) - landfall in Charlotte County, Florida with winds of 150 mph. Hurricane Dennis (2005) - struck the island of Cuba as a Category Four hurricane.

Category
Category 5 Storm
Sustained Winds: ≥ 155 mph ≥ 250 km/h
Estimated Storm Surge: < 27.17 inHg < 920 mbar
Potential Damage: Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. All shrubs, trees, and signs blown down. Complete destruction of mobile homes. Severe and extensive window and door damage.
Examples: Only 3 Category Five Hurricanes have made landfall in the United States since records began: The Labor Day Hurricane of 1935, Hurricane Camille (1969), and Hurricane Andrew (1992).
NOAA - National Hurricane Center (NHC): The Saffir-Simpson Hurricane Scale