Formation | Diagram

A mesocyclone forms when warm, moist air meets cold, dry air. This tenses up into strong rotation. This combo creates powerful clouds that slowly spin. This is what creates the "mesocyclone" as we know today. This is the roation needed to create a tornado.

The mid level winds enter the mesocyclone with a strong force. The winds get disoriented by the mesocyclone and make a shift downwards towards the ground. This creates a "Rear flank downdraft" which brings the mesocyclone down to the ground, forming a tornado.

The overshooting top is where the mesocyclone exits, which is towards the top. This releases strong winds on the top of the supercell. This is why you can see debris flying in a upwards direction in a tornado. Usually this is not visible from the ground when your looking up at a supercell.

The upper level winds give the signature structure to the supercell. This makes it very easy to identify as a supercell storm. These strong winds shoots through the top, right below the over shooting top causing the anvil shape.

The foward flank downdraft gives the storm the direction it is moving in. This also helps bring the tornado down, but however it moves the storm along with it. 

This part of the super cell brings strong winds straight into the bottom. These winds either add more power to the meso cyclone or gets disoriented and adds another direct down draft that brings down the funnel. This is the main force that bring the funnel cloud down. This also can move the tornado. As you can see in the diagram next to this text, this causes a gust front which is the direction the storm moves.