How do hurricanes form?

Hurricanes are tropical cyclones that typically occur in the in Atlantic and Pacific oceans.  When ocean waters warm, water evaporates into the lower atmosphere.

According to Wikipedia, hurricanes “…feed on heat released when moist air rises, resulting in condensation of water vapor contained in the moist air.”  The released heat is called “heat of condensation”.  The released heat is “distributed vertically around the center of the storm. Thus, at any given altitude (except close to the surface, where water temperature dictates air temperature) the environment inside the cyclone is warmer than its outer surroundings.”

In a thunderstorm, the released heat of condensation is carried away by surrounding strong winds (also called “wind sheer”).  But when the winds are calmer, the heat can continue to build in the vicinity, creating an intense low-pressure system — leading to the birth of a hurricane.

Because of a phenomenon called coriolis force, hurricanes have counterclockwise rotation in the Northern Hemisphere and clockwise rotation in the Southern Hemisphere.
http://www.usatoday.com/weather/resources/basics/coriolis-understanding.htm

For more information on what causes hurricanes and how hurricanes form, visit the following Wikipedia reference page:  http://en.wikipedia.org/wiki/Tropical_cyclone

Animated hurricane image from the National Oceanic and Atmospheric Administration (NOAA)