To understand what energy is it might help to understand quantum mechanics.

Now for a technical definition, Quantum mechanics is a theory of matter and energy that explains in particular, energy is absorbed and released, in small quantities (or quanta), and that all matter displays both wavelike and particle-like properties, especially when viewed at subatomic scales. That definition was taken from the American heritage science dictionary, and only slightly edited. 

So energy is in constant flux or in other terms, it's continually moving and changing, which can be seen when you look at it on teeny, teeny tiny levels. That's easier to understand, right dictionary?