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The concept of enthalpy is killing me. From what I read enthalpy is the useful energy obtained from a closed system. What the hell does that even mean? What is "useful" energy?

Lets say I apply 10 megajoules of energy to closed mechanical system. Lets say it takes 6 megajoules to turn a turbine in the system. I have 4 megajoules left over. First of all the whole 10 megajoules is useful work isn't it or is it only the 6 megajoules that turns the turbine that is considered to be useful work? Therefore what wn'ould the enthalpy of this system be?

I haven't studied thermodynamics at all but this enthalpy word keeps popping up in chemistry and its driving me insane. I need to figure out what the hell it means.
 
A better example would be enthalpy of a chemical system. I know what enthalpy change is. Its just the change in energy during a chemical reaction.

Does enthalpy just mean the potential energy of the molecule? In an endothermic reaction for example the products will have more energy than the reactants. There was a positive enthalpy change. The products are now stable. Is this energy the product possess stored in the form of chemical bonds? How does enthalpy tie into this?