What is a mechanism?
A mechanism is a combination of a number of bodies assembled in such a way that the motion of one causes constrained and predictable motion to the others.
The purpose of mechanism is to transform motion into some desirable pattern. It typically develops very low forces and transmits little power. If the forces or energy levels within the device are significant, it is considered a machine; if not, it is considered a mechanism.
For example, a slider crank mechanism converts reciprocating motion of slider into rotary motion of crank or vice-versa. However, when it is used as an automobile engine by adding valve mechanism, it becomes a machine which converts the available energy force on the piston into the desired energy, torque of the crank shaft. This torque is used to move a vehicle.
Some examples of common mechanisms are a pencil sharpner, a camera shutter, an analog clock, a folding chair, an adjustable desk lamp, and an umbrella.