The Fujita Scale is a measure of how powerful a tornado is, it is a scale between 0-5, 0 being barely any damage to 5 being very damaging. There are two Fujita scales the original and the Enhanced.
Original Fujita Scale
Enhances Fujita Scale
The Enhanced Fujita scale uses EF-0-5 rather than F-0-5 with the original Fujita, the EF scale is more commonly used now days while the original being considered obsolete.