A tornado is defined as a violently rotating column of air in contact with the ground and pendent from a cumulonimbus cloud.
Photograph by: Marshall
They can be categorized as "weak", "strong", and "violent"; with weak tornadoes often having a thin, rope-like appearance, as exhibited by this tornado near Dawn, Texas. About 7 in 10 tornadoes are weak, with rotating wind speeds no greater than about 110 MPH. (looking west from about 1 mile.)