Scud clouds are found at the base of a thunderstorm but are actually detached from the storm. They can move up, down, and side to side, but they do not spin.
The Shelf Cloud hangs over the base of a thunderstorm. Sometimes fragments of the shelf can extend down like fingers. These cloud fragments are very commonly mistaken for funnels. They can also have a lot of motion to them, but they do not spin and are not dangerous.
A Wall Cloud hangs down below the base of a thunderstorm below the Shelf. Usually the Wall is the lowest point of a rotating supercell thunderstorm and is an indication of danger. It's rotation is very slow and often undetectable to the human eye in real time. Most of the large and violent tornadoes come out of the Wall Cloud.
This is the beginning stage of a tornado. Funnel clouds are rotating columns of air that visibly extend down from the thunderstorm and they can occur at any location near the bottom of the storm. Often times funnels do not reach the ground but are an indication that a tornado is trying to form. Their rotation is very tight and is easier to identify.
This is violently rotating column of air that extends from the thunderstorm to the ground. The entire vortex is not always condensated or visible, but it is always in contact with the surface. There is usually a puff of debris from dirt or structures at the point of contact. It is not a tornado unless that contact is being made.