The lever is a beam or a bar that is supported by one point called a fulcrum. There are three types of levers. They are class 1, class 2, and class 3. The levers are classified by where their fulcrum point is located. Where the fulcrum point is located will also change the length of the load arm and the effort arm. Each lever has it's own features which can affect mechanical advantage. Mechanical advantage is the amount that a machine can multiply the input force. Some examples of a lever are: a crowbar, your arms, and teeter totter. Click here for a video intro to levers.
Links
Intro
Load Arm and Effort Arm
Levers and Friction
Classes Of Levers
Human Levers
Wheels
Bibliography