Tornado Watches and
Warnings

Tornado watches and warnings are issued by the
National Weather Service or the National Severe Storms Lab, which
constantly monitor atmospheric conditions over the United States.

Tornadoes are very destructive, so it's
important to know when one may form so you can take shelter. Forecastors
at the National Weather Service are always on the lookout for developing
storms. Even though nobody knows how tornadoes form,  they do know
the conditions when they are most likely to form.

What's the different between a

tornado watch and a
tornado warning?

      
TORNADO WATCH

A tornado watch
means that conditions are favorable for producing a particular weather
event, but that it has not formed yet. Watches alert the public that
they need to pay closer attention than usual to the weather, just to be
safe. A tornado watch is issued when atmospheric conditions could cause
tornadoes to form, although none have formed yet.

   
TORNADO WARNING

A tornado warning means
that a particular weather event has formed and that it threatens the
area under the warning. A tornado warning means that a tornado is
actually present and moving in the warn vicinity. Residents should take
immediate shelter in a tornado warning.

Source:

http://www.tapestryweb.org/tornado/watches.html