When two or more simple machines are put together, they make a compound machine a wheel barrow is made up of several simple machines.
Before getting into compound machines, consider what makes something a simple machine.
Simple machines have no (or just a few) moving parts - hence their name. They do not amplify the force you apply. Instead, they simply change the direction of the force. For example, the curved edges along the screw increases the distance the force travels, allowing you to use less force to do the same task. Mechanical advantage, or MA, measures how easy it is to do work. This is the measurement of the force gained by using the simple machine. In a simple machine, the mechanical advantage, or the magnification of the force, is only one. However, because it changes the direction of the force, it makes work feel easier.
The six simple machines are levers, pulleys, wedges, wheels & axles, inclined planes, and screws. For example, a set of stairs or ramps is an inclined plane. The cutting edge of your dinner knife is a wedge. Skateboards work thanks to their two sets of wheels and axles.
Consider some of the work simple machines do. Imagine simply pushing on the top of a screw to try to get it into a piece of wood. Pushing down straight through the screw would be difficult. However, when the screw is turned, the direction of the force changes, and follows along the curved edges so that the screw can go into the wood. Or, think about pushing on one end of a stick while the other is wedged under a rock. Forces applied at one end, and the object moves at the other end of the stick.