The strength of tornadoes is measured by the Enhanced Fujita Scale, which places a tornado into six categories based on wind speed and damage caused.
Unlike hurricanes, which are classified prior to making landfall, the strength of a tornado isn't classified until after it has already touched down.
Once a tornado touches down, a team of meteorologists from the National Weather Service will physically go to the site of the damage and conduct a survey to determine how strong the tornado was. It's from this survey that the tornado receives its official classification:
65–85 mph winds
86–110 mph winds
111–135 mph winds
136–165 mph winds
166–200 mph winds
Winds of more than 200 mph
The Enhanced Fujita Scale is an updated version of the original Fujita Scale that was introduced in 1971 by Tetsuya Theodore Fujita.