A cam is a wheel on an axle that is offset from the centre of the wheel, so that when it rotates on its axis the majority of the wheel will be above or below the axle. They typically work with a cam follower that follows the cam and transfers elsewhere. The cam follower is usually a rod touching the top of the cam that moves up and down as the cam raises and lowers; the cam pushes the cam follower up, and gravity pulls the cam follower back down. The cam follower is supported by a shaft that holds it vertical, and has a smooth end to keep the mechanism from jamming. To summarise, a cam is used to turn circular motion into linear motion, called reciprocating movement.
Gears are used to control the speed of rotation or to change the direction of rotation. They're circular objects with teeth sticking out on the edge. The first gear being turned is called an input gear, and the gear that is then caused to be turned is called the output gear. When the input gear turns, its teeth interlock with the teeth on the output gear, causing it to rotate in the opposite direction.
Gears with unequal numbers of teeth alter speed of rotation. If the input gear has fewer teeth than the output gear, the speed of rotation slows down, which is called stepping down. The reverse of this is stepping up. Stepping down produces more power at a slower speed, which can make turning a crank easier, while stepping up produces less power at a faster speed.
Pulleys work the same way gears do, but they don't directly touch each other. They're circular wheels with grooves on the edges, like narrow spools. They're connected by a belt around them, which is held in place by the grooves. Pulleys can also have teeth, while the belt has holes for the teeth to go through to ensure that the belt doesn't slip off; this can be seen in a bicycle chain. An advantage of pulleys over gears is that they can be used far apart, and can also be used to change the direction of the motion. They function in the same way that gears do, but the differences between the two wheels are determined by their diameters rather than number of teeth.
Levers are used to apply force or transfer it. They're usually just a slat or rod that pivots around a point called a fulcrum. Important terms to remember include the load, which refers to whatever you want to manipulate with the lever, and the effort, which is the point at which force is applied.
There are three kinds of levers, known as orders, which are defined by whether the fulcrum, load, or effort is in the middle. A first order lever has a fulcrum between the load and effort, and works like a see-saw, in which the effort is the side of the see-saw going down and the load is the side going up. You can produce a small motion from a large force by moving the fulcrum closer to the load, while the opposite happens if you move the fulcrum closer to the effort. A second order lever has a load in the middle, and works like wheelbarrow in which the fulcrum is the wheel and the effort is the handle you lift from. These can be used to increase force. A third order lever has the effort in the middle and works like a shovel in which the fulcrum is the handle on which you push down as you lift from the effort in the middle. These can be used to increase movement.