Those are all terribly important questions that really do require that an attempt be made to answer before getting into the subject. Without such a quantification, an abstract concept becomes all the more confusing. Discussions on the subject usually devolve into a woo-woo fest, without really getting anywhere.
Thus, I'll give it a shot:
What is energy work?
My understanding of energy work is that it is the process of tapping into and manipulating the subtle flow of natural energies around and within us. With time, practice, and skill, one can use those vibrations to cause changes in line with one's own desires.
In essence, energy work, not unlike the idea of psionics, involves the use of intention which directly modifies and encodes the resource at the center of this discussion. Reiki is only one such example of a practice involving the use of energy work.
How do you quantify "energy"? What is it?
When I use the word "energy" to describe an essence used in the aforementioned way, I am referring to the subtle vibrations and fields which permeate any and all things. Not unlike the Force described in Star Wars, its a permeable, translucent thing that one with the appropriate level of awareness can feel, sense te qualities of, and use.
How is it useful?
This depends on you. Healing through energy work is one such use. Harming too. It can be used to create barriers, shields, spiritual creatures, purify things, contaminate them, create repositories of information, and so on and so forth. Its an infinitely useful resource, but it requires a great deal of skill to use effectively.
How does one utilize it effectively?
In a word, focus. Energy flows where the mind goes. Not sure who first said that phrase, but they weren't wrong.
Thus sums up my current views and sets the stage for discussion on the subject.