Machine – It is a device which has relative motion between its members or links . A machine converts available Energy into useful work. The members of machines transmit motion and forces. Examples – Bike, Lathe etc
Structure – A structure has no relative motion between its members or links. It cannot convert the available energy into useful work. It is meant to take loads only. Examples – Frame, Bridge etc.
Mechanism – It is used to transmit motion. It has definite motion between various links. A simple mechanism has four links while compound mechanism consists of more than four links. Example – Typewriter