Short answers, pulled from the story.
A simple machine is a mechanical device that changes the direction or magnitude of a force. It uses mechanical advantage, also called leverage, to multiply force, and the term usually refers to the six classical devices defined by Renaissance scientists.
The six classical simple machines are the lever, the wheel and axle, the pulley, the inclined plane, the wedge, and the screw. Renaissance scientists defined this set.
The Greek philosopher Archimedes originated the idea of a simple machine around the 3rd century BC and discovered the principle of mechanical advantage in the lever. His remark, "Give me a place to stand on, and I will move the Earth," expressed that there was no limit to the force amplification mechanical advantage could achieve.
A screw is self-locking because friction between its large sliding surfaces prevents the load force from turning it backward. A machine is self-locking if and only if its efficiency is below 50 percent, which depends on both the static friction between parts and the ideal mechanical advantage.
The mechanical advantage of a compound machine equals the product of the mechanical advantages of the simple machines that form it. A compound machine connects simple machines in series, with the output force of one providing the input force to the next, as in a bench vise or a gear train.
In 1586 the Flemish engineer Simon Stevin derived the mechanical advantage of the inclined plane, adding it to the list. In 1600 the Italian scientist Galileo Galilei worked out the complete dynamic theory in Le Mecaniche, showing the machines' mathematical similarity as force amplifiers and explaining that they transform energy rather than create it.
Modern machine theory analyzes machines as kinematic chains composed of elementary linkages called kinematic pairs. Franz Reuleaux, who studied over 800 elementary machines, showed that levers, pulleys, and wheels and axles are bodies rotating about a hinge, while inclined planes, wedges, and screws are blocks sliding on a flat surface, making the joints the primary elements.