The AP has released a very handy explanation of the Saffir-Simpson Hurricane Scale:
The Saffir-Simpson scale of a hurricane's intensity is used to estimate the potential property damage and coastal flooding. The scale is determined by wind speed, since storm surge sizes depend on the slope of the continental shelf.
Category 1: Winds 74-95 mph. Storm surge 4 to 5 feet above normal. Damage primarily to unanchored mobile homes, shrubbery and trees. Some damage to poorly constructed signs and piers.
Category 2: Winds 96-110 mph. Storm surge 6 to 8 feet above normal. Some roof, door and window damage of buildings. Considerable damage to mobile homes, small watercraft, trees, poorly constructed signs and piers. Flooding of coastal and low-lying areas.
Category 3: Winds 111-130 mph. Storm surge 9 to 12 feet above normal. Some structural damage to small homes. Mobile homes destroyed and large trees blown down. Coastal flooding destroys smaller structures and floating debris damages larger structures. Terrain lower than 5 feet above sea level may flood as far as 8 miles inland. Hurricane Rita, which struck last September along the Texas-Louisiana line, was a Category 3 storm.
Category 4: Winds 131-155 mph. Storm surge 13 to 18 feet above normal. Wall failures and roof collapses on small homes, and extensive damage to doors and windows. Complete destruction of some homes, especially mobile homes. Major coastal flooding damage. Hurricane Katrina was a Category 4 storm as was Hurricane Ivan, which made landfall near Gulf Shores, Ala., last September and Hurricane Charley, which hit the Florida Gulf Coast near Fort Myers last August.
Category 5: Winds greater than 155 mph. Storm surge greater than 18 feet above normal. Complete roof failure on many homes and industrial buildings. Smaller buildings and mobile homes blown over or completely blown away. Major damage to lower floors of all structures located less than 15 feet above sea level and within 500 yards of the shoreline. Massive evacuation of residential areas on low ground within 5 to 10 miles inland may be required. Last Category 5 storm to hit the United States was Hurricane Andrew in 1992.