For questions relating to the energy required for or produced by reactions, including questions of endothermicity/exothermicity, bond enthalpy, etc.
Energy, in physics, is defined as "an indirectly observed quantity that is a prerequisite for performing work". Exactly what it is is an open question of physics.
In chemistry, energy most often manifests as thermal energy or heat. Chemical bonds store a given amount of energy and also require the addition of more energy from the outside environment to break these bonds. This energy that is required to break the bonds between two atoms of a compound and form another bond is known as its "enthalpy of formation" or "heat of formation", and is typically measured either in kilojoules per mole (the Joule is the SI unit favored in physical sciences) or kilocalories per mole (more easily obtained by thermal measurement). The units can be easily converted; 1kcal/mol = 4.184kJ/mol. Chemical bonds with lower enthalpies of formation are characterized as less stable; they have more energy inherent in the bond and so will more easily break that bond to form another. Bonds with higher enthalpies of formation are more stable.
Chemical reactions that break less stable bonds to form more stable ones release the additional energy, typically in the form of heat. These reactions are known as "exothermic". By contrast, chemical reactions that break more stable bonds to form less stable ones require the addition of energy from the environment in which the reaction takes place; these are known as "endothermic". The total energy production or consumption inherent in a chemical reaction can be calculated by computing the sum difference of the enthalpies of the existing bonds broken and the new bonds formed. The resulting quantity, typically also in kJ/mole or kcal/mole, is the "heat of reaction".
Chemical energy can also manifest itself as kinetic or electrical energy. Reactions involving the transfer of electrons and/or protons, inducing a charge imbalance in one or more of the reactants, can reach equilibrium by transferring electrons, creating an electrical current. Highly exothermic reactions, especially those that create gaseous products, can also produce kinetic energy as the newly-formed, highly-compressed gases rapidly expand to equalize pressure with the surrounding environment.