The following differences between a machine and a structure are important from the subject point of view:

Structure
1. the member of structure do not move relative to one other.
2. Structure Do not transform energy in to the useful work.
3. the member of structure transmit forces only.
4. it dose not have mechanism.
5. E,g: Railway Bridge, truss, Machine frames.

Machine
1.  the part of machine move relative to one another.
2. m/c Transform available energy into useful work
3. The link of m/c made transmit both power relative motion and forces.
4. M/c can have one or more mechanism
5. E.g : length m/c , Ic engine .