As you probably know, there are 6 simple machines:

1) Lever

2) Pulley

3) Wheel and Axle

4) Inclined Plane

5) Wedge

6) Screw

In the nineteenth century, the German mechanical engineer Franz Reuleaux (1829 - 1905) postulated that the 6 simple machines could be placed into 2 groups:

    Ⅰ) the lever, the pulley and the wheel and axle
      The "lever family" is grouped together because they function around a central point:
      the fulcrum, the pulley axis and the axle.

    Ⅱ) the inclined plane, the wedge and the screw
      The "inclined plane family" is grouped together because they function using angles.

A simple machine is something that can change the direction of a force, change the magnitude (amount) of a force or both.

In a simple machine, effort is the unchanged force (or input force) and load is the changed force (or output force).

When a simple machine changes the magnitude of a force, you are gaining what is known as a mechanical advantage, which equals the output force divided by the input force.

So, for example, if you are pressing fifty pounds on a lever to move a 200 pound boulder you have a mechanical advantage of 4. (200 ÷ 50 = 4)