Although they may seem very different, wheels and gears are closely related to levers. The wheel is essentially a rotating lever. Think of the diagram of the lever above as being bent into a circle. The center of the circle is the fulcrum. In math we call this the center point.

Wheel "A" is a circle. It is larger than wheel "B". The black circle. You will notice that "A" is larger than "B". The diameter of "A" is larger than "B". If you rotate "B"(the axle) it will move wheel "A". Since "B" is smaller than "A" and moves in a smaller circle than "A" you will use very little input you get a large output. The large ratio of the wheel radius to the axle radius is what makes it easy to turn the wheel at it's rim.