Chemical reactions are changes in the structure of molecules. Such reactions can result in molecules attaching to each other, forming larger molecule or they may involve the opposite process, a molecule breaking apart, forming two or more, smaller molecules.
Changes in the composition of the nuclei of atoms are called nuclear reactions, and are NOT considered chemical reactions.
A chemical reaction will almost always involve a change in energy, historically measured in terms of heat. The amount of energy difference between the before and after of a chemical reaction can be calculated theoretically using tables of data. Let us take the example of the reaction CH4 + 2O2 > CO2 + 2H2O (combustion of methane in oxygen). By calculating the amount of energy required to break all of the bonds on the left hand side of the equation (before the reaction), and the amount of energy given off by making all the bonds on the right hand side of the equation (after the reaction), we can calculate the energy difference. This is referred to as ΔH, where Δ (Delta) is the capitalised version of the fourth letter of the Greek alphabet. Δ stands for difference in chemistry, and the H refers to heat, a measure of energy. ΔH will usually be given in KJ, or thousands of Joules. If this amount is negative for the reaction, then energy has been given off (i.e. less energy is required to break all of the bonds on the left than is gained by making all of the bonds on the right). This type of reaction is referred to as exothermic (literally heat without, or outside heat). An exothermic reaction is more favourable, and more likely to occur. Our example reaction is exothermic, as we know from everyday experience, burning gas in air gives us heat.
It is possible to have a reaction which has a positive ΔH. This means that the reaction will require an outside input of energy to work (i.e. more energy is required to break all of the bonds on the left than is gained from making the bonds on the right). This type of reaction is called endothermic (literally heat within).