Simple machines allow people to make tradeoffs that reduce the amount of work necessary to perform a task. Simple machines may convert one type of force to another, change the direction of an applied force or trade distance travelled for force applied.
For example, people can use inclined planes to lift a heavy load more easily. Rather than lifting a heavy box straight up from the ground, an inclined plane allows the person to raise the load gradually. The trade off in this example is that the load must be moved a greater distance than if it were lifted straight up.
As another example, a pulley changes the direction of applied force. For example, it may be difficult to push a heavy load up a hill, while pulling it is relatively easy. By attaching the load to a rope, running the rope through the pulley and back down to the operator, the load can be pulled up more easily than it could be pushed up.
Levers work by allowing the user to trade the distance travelled for increased force. For example, a high jumper may only be able to leap a few feet off the ground, but a pole vaulter, who uses the pole like a lever, can travel many times higher with no more force applied.