These definitions come from the NWS Jetstream Online Shool for Weather --- Glossary of Weather terms used.
It is linked backed to the Jetstream home page at
http://www.srh.weather.gov/jetstream/index.htm which I believe you may enjoy.
Good Luck -- Great questions
Bill A N2JAI
Skywarn Coordinator Cape May NJ
http://groups.yahoo.com/group/CMCSPOTTERS/
Warning
A product issued by NWS local offices indicating that a particular weather hazard is either imminent or has been reported. A warning indicates the need to take action to protect life and property. The type of hazard is reflected in the type of warning (e.g., tornado warning, blizzard warning). See short-fuse warning.
Watch
An NWS product indicating that a particular hazard is possible, i.e., that conditions are more favorable than usual for its occurrence. A watch is a recommendation for planning, preparation, and increased awareness (i.e., to be alert for changing weather, listen for further information, and think about what to do if the danger materializes).
Flood Watch (FFA)
This watch is issued by a local National Weather Service Office to indicate that there ia a potential of flooding in or close to the watch area. Those in the affected area are urged to be ready to take action if a flood warning is issued or flooding is observed. In flooding, the onset of flooding take place much slower (usually greater than 6 hours) than a flash flood. This type of flooding usually occurs with "train echoes" or slow moving thunderstorms, and can also occur with synoptic scale systems that last a relatively long period of time and encompass a large area. They are usually issued up to 12 hours prior to the possible flood event. These watches can vary in size depending on the size of the meteorological event.
Flood Warning (FLW)
This warning signifies a longer duration and more gradual flooding of counties, communities, streams, or urban areas. Floods usually begin after 6 hours of excessive rainfall. They are issued by the local National Weather Service Forecast Office for 6 hours or less.
Tornado Watch (SEL)
This is issued by the National Weather Service when conditions are favorable for the development of tornadoes in and close to the watch area. The watch is in a form of a rectangle whose size and orientation varies depending on the weather situation. They are usually issued for a duration of 4 to 8 hours. They normally are issued well in advance of the actual occurrence of severe weather. During the watch, people should review tornado safety rules and be prepared to move a place of safety if threatening weather approaches.
Tornado Warning (TOR)
This is issued when a tornado is indicated by the WSR-88D radar or sighted by spotters; therefore, people in the affected area should seek safe shelter immediately. They can be issued without a Tornado Watch being already in effect. They are usually issued for a duration of around 30 minutes.
A Tornado Warning is issued by your local National Weather Service office. It will include where the tornado was located and what towns will be in its path. If the tornado will affect the nearshore or coastal waters, it will be issued as the combined product--Tornado Warning and Special Marine Warning. If the thunderstorm which is causing the tornado is also producing torrential rains, this warning may also be combined with a Flash Flood Warning.
After it has been issued, the affected NWFO will followed it up periodically with Severe Weather Statements. These statements will contain updated information on the tornado and they will also let the public know when warning is no longer in effect.