NOAA offers us this excellent description of the Fujita Scale and helps you to understand more about how tornadoes work, as well as what the Fujita scale measures.
F0     40 to 72 MPH     Some damage to chimneys, TV antennas, roof shingles, trees, and windows.     29% of all tornadoes

F1     73 to 112 MPH     Automobiles overturned, carports destroyed, trees uprooted     40%

F2     113 to 157 MPH     Roofs blown off homes, sheds and outbuildings demolished, mobile homes overturned.     24% of all tornadoes

F3     158 to 206 MPH     Exterior walls and roofs blown off homes. Metal buildings collapsed or are severely damaged. Forests and farmland flattened. 6% of all tornadoes

F4     207 to 260 MPH     Few walls, if any, standing in well-built homes. Large steel and concrete missiles thrown far distances.     2% of all tornadoes

F5     261 to 318 MPH     Homes leveled with all debris removed. Schools, motels, and other larger structures have considerable damage with exterior walls and roofs gone. Top stories demolished  1% of all tornadoes