What causes a tornado, and do they always come from super cells?

Basically its when Hot and Cold Air masses combine. Mostly occurs in central USA

For one instance, lets say that a cold front is coming in to the central U.S, where a high pressure system is ruling the area with warmth and dryness. Here's how I see it: The air masses collide, causing the warm air to push up, and the cold air to push down. There, in the process, if the two different winds turn with height and different direction, a horizontal "tube" is created. With a sudden and "sharp" updraft, the "tube" is tilted vertically. The winds now begin to spin faster and faster, as the wall cloud, tail cloud, and (now a funnel cloud) form. Tornadoes can range from short to long periods of time on the ground.

Tornadoes always come from super cells. Wait, let me re-phrase that, super cells are the only producers of tornadoes, but not all super cells produce them.

Most tornadoes are the result of strong wind shear (winds changing in both speed and direction with height) in the low to mid levels of the atmosphere, combining with thunderstorms. The shear induces a sort of horizontal roll in the lower atmosphere, and when a storm forms or passes over this area it can pick up the horizontal roll and tilt it vertically, producing a mesocyclone which characterizes a supercell storm. Not all supercell storms produce tornadoes (actually only a small percentage do) and likewise not all tornadoes are from supercell storms.