From About.com. Excerpt:

The birth of a hurricane starts as a low pressure zone and builds into a tropical wave of low pressure.

In addition to a disturbance in the tropical ocean water, the storms that become hurricanes also require warm ocean waters (above 80°F or 27°C down to 150 feet or 50 meters below sea level) and light upper level winds.

A tropical wave grows in intensity and then may grow to become an organized area of showers and thunderstorms known as a tropical disturbance. This disturbance becomes an organized area of tropical low pressure that is called a tropical depression based on cyclonic winds (counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere). A tropical depression's wind speed must be at or below 38 miles per hour (mph) or 62 km/hr when averaged out over one minute. These winds are measured at 33 feet (10 meters) above the surface.

Once average winds reach 39 mph or 63 km/hr then the cyclonic system becomes a tropical storm and receives a name while tropical depressions are numbered (i.e. Tropical Depression 4 became Tropical Storm Chantal in the 2001 season.) Tropical storm names are preselected and issued alphabetically for each storm.

There are approximately 80-100 tropical storms annually and about half of these storms become full-fledged hurricanes. It is at 74 mph or 119 km/hr that a tropical storm becomes a hurricane. Hurricanes can be from 60 to almost 1000 miles wide. They vary widely in intensity; their strength is measured on the Saffir-Simpson scale from a weak category 1 storm to a catastrophic category 5 storm. There were only two category 5 hurricanes with winds over 156 mph and a pressure of less than 920 mb (the world's lowest pressures ever recorded were caused by hurricanes) that struck the United States in the 20th century. The two were a 1935 hurricane that struck the Florida Keys and Hurricane Camille in 1969[Note: Hurricane Andrew has been upgraded from Category 4 to 5 since this was written]. Only 14 category 4 storms hit the U.S. and these included the nation's deadliest hurricane - the 1900 Galveston, Texas hurricane and Hurricane Andrew which hit Florida and Louisiana in 1992.

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