What is enery? I understand that there are different types of energy and that it is either absorbed or released during chemical reactions but what exactly is energy? Is it some kind of particle, wave, heat?

Energy is the capacity to do work. It's a property of objects or systems; ultimately it's a bookkeeping convenience, because it's a conserved quantity in isolated systems. If one does work on a mechanical system the kinetic energy changes, and we notice that there are ways of storing this energy in a reversible way related to the position, so we have potential energy, and then there are thermodynamic equivalents and we can convert mechanical work into thermal energy and back again.

 

("What is energy" as a metaphysical question isn't addressed by science, because it's metaphysics)