What is the difference between a "flood" and a "flash flood"?
Flood: An overflow of water onto normally dry land. The inundation of a normally dry area caused by rising water in an existing waterway,
  such as a river, stream, or drainage ditch. Ponding of water at or
  near the point where the rain fell. Flooding is a longer term event
  than flash flooding: it may last days or weeks.
  
  Flash flood: A flood caused by heavy or excessive rainfall in a short period of time, generally less than 6 hours. Flash floods are
  usually characterized by raging torrents after heavy rains that rip
  through river beds, urban streets, or mountain canyons sweeping
  everything before them. They can occur within minutes or a few hours
  of excessive rainfall. They can also occur even if no rain has fallen,
  for instance after a levee or dam has failed, or after a sudden
  release of water by a debris or ice jam. 
  
  Source: National Weather Service Weather Forecast Office