A MACHINE is a device whose mission is either to facilitate the performance of a task or to cut down on the amount of force needed to perform that task.
A SIMPLE MACHINE takes a force applied to it and convert it into a more useful output force. Example of simple machines are levers,wheels (pulleys), inclined planes (screws).
A MECHANISM is more complex. It could be a combination of different machines. It is a device that transmit and convert forces and motions from a input energy to an output energy. It enables us to carry out certain tasks with greater comfort and less effort. Examples: gears, crank-link-slider, crankshaft, cam, eccentric cam...
We can classify mechanisms in two parts:
to transmit of motion ( lineal motion , rotary motion)
to convert to motion ( rotary to lineal, rotary to reciprocating)