As almost anyone who took Physics can tell you (for the first two or three weeks after the course), there are 3 "simple machines" on which all others are based. The "Big Three" are (in no particular order):

• The Wheel - consists of a wheel turning about an axle; serves to replace sliding friction (wheel to ground) with rolling friction (axle against wheel's hub) which is significantly lower due to a smaller surface area and less relative velocity between the contact surfaces. Choosing the materials and radii of wheels and axles allows the designer to greatly reduce friction losses e.g. a skateboard.
• The Inclined Plane - consists of any surface that is not horizontal or vertical placed in proximity to one that is; serves to transfer force at an angle, so that a small amount of force with a large displacement can be transferred in a perpendicular direction as a larger force with a small displacement. e.g. a doorstop, which exerts great vertical force from a gentle kick. All blades are inclined planes.
• The Lever - Consists of a fulcrum and a lever, and transfers force at a distance, repaying you with multiplied force for a trade-off of distance, or vice versa. e.g. a crowbar, with 6cm from fulcrum to business end and a 60cm handle will pry with 10x the force you put into it, but the business end will only move 1/10 the distance.

Many other things we think of as "simple" machines are combinations of the three basic principles. For example:

• A pulley is a wheel, and if you try to hoist a load vertically by pulling the rope at a shallow angle, the rope is an inclined plane.
• A corkscrew is a long, continuous inclined plane wrapped around a central shaft. If it's a typical corkscrew for opening wine bottles, the T-handle at the top is a lever.
• A toothed gear is a wheel, but functionally it is a lever, with its fulcrum at the axis.

And putting it all together, you have:
• A pizza cutter is a wheel, whose sharp edge is a pair of inclined planes back-to-back, and whose handle is a lever. The butt of your hand applies downward force; your fingers form the fulcrum (by pulling upward on the grip) and the handle magnifies the force. This magnified downward force causes the inclined planes to force the pizza on the left of the blade away from the pizza on the right. Any forward force makes the wheel rotate, and the tool moves along the surface to be cut.

All simple machines conserve1 mechanical energy (the mathematical product of force and distance) but allow the user to trade one for the other. Every machine works on these principles; exceptions are machines that consume or produce non-mechanical energy (chemical, electrical, etc.)--electric generators and motors, internal combustion engines, and so forth. As long as the energy in the "ideal" (lossless) system is conserved (or in a "real" system, once entropy and friction take their slices), the mechanical parts are made from these three components in combinations and permutations far too varied to list here.

1. Neglecting, of course, that friction will always convert a small amount of the system's mechanical energy into thermal energy.