What energy is
Work and energy are two views of one thing. Work is what gets done; energy is what makes it possible to do.
Energy
energy = the ability to do work
Anything that can do work has energy, and it has exactly as much energy as the work it can do.
The unit is the joule again
Because energy is measured by the work it can do, it cannot be measured in anything other than the unit of work. Energy is measured in joules, exactly as work is.
One unit for both
This is not a coincidence or a convenience. If energy is the capacity to do work, then a store of 60 J of energy simply means a store that can do 60 J of work — the same number, counted once from each side.
Why we want energy at all
We need energy for one reason: to get work of various kinds done. The next page sorts the forms it comes in.
Energy is measured by what it can do
Energy is the ability to do work, and both are measured in joules (J).