The relationship between Work and Energy can be shown by:
Work is the change in Kinetic Energy.
Definition
Interpreted differently, energy is the “currency” which we “spend” to do work.
The work-energy theorem states that:
The net work done by the forces acting on a body is equal to the change in the kinetic energy of the body.
The work-energy theorem is an invariant law, meaning that it is the same in all reference frames.
Also, the Work-Energy theorem only applies to individual particles and not Systems of Particles. Each part of the object should behave in exactly the same way.
For example, in a car crash, different parts of the car act differently in response to the crash and there is internal energy moving with the bending of the car, so we cannot apply the Work-Energy Theorem exactly.
This also applies to friction; friction is complicated and is generally not correct to apply the work-energy theorem to particles undergoing frictional forces.