The F scale was developed by Dr. Theodore Fujita in order to relate the degree of damage due to high winds, to the intensity of the wind. This scale rated tornadoes from a F0 to F5, with each category representing a wind speed range. Since wind speed is not easily measured, it was estimated by evaluating the damage after a tornado or high wind event. The fallacy in this method was that the damaged structures building materials and strength were not considered.
The EF scale was developed and is a more precise way to classify wind events. This scale uses 28 different types of damage indicators, such as building materials, when evaluating the damage after a tornado or other strong wind.
For a very detailed explanation of the EF Scale click here.