There are several types of chemical reactions. Synthesis consists of 2 or more individual molecules coming together and forming a new substance. Decomposition is the opposite of synthesis, and in this type reaction individual atoms are no longer bonded. In single displacement reactions, one of the atoms' formula is exchanged with another one's. In double displacement reactions, both reactants' bonded atoms are exchanged.
A chemical reaction does not change the nucleus of the atom in any way, only the interaction of the electron clouds of the involved atoms. (Changes in the composition of the nuclei of atoms are called nuclear reactions, and are not considered chemical reactions, although chemical reactions may follow a nuclear transformation.)
A chemical reaction almost always involves a change in energy, conveniently measured in terms of heat. The energy difference between the "before" and "after" states of a chemical reaction can be calculated theoretically using tables of data (or a computer). For example, consider the reaction CH4 + 2 O2 → CO2 + 2 H2O (combustion of methane in oxygen). By calculating the amounts of energy required to break all the bonds on the left ("before") and right ("after") sides of the equation, we can calculate the energy difference between the reactants and the products. This is referred to as ΔH, where Δ (Delta) means difference, and H stands for heat, a measure of energy. ΔH is usually given in units of kJ, or thousands of joules. If ΔH is negative for the reaction, then energy has been released. This type of reaction is referred to as exothermic (literally, outside heat, or throwing off heat). An exothermic reaction is more favourable and thus more likely to occur. Our example reaction is exothermic, which we already know from everyday experience, since burning gas in air gives off heat.
A reaction may have a positive ΔH. This means that, to proceed, the reaction requires an input of energy from outside. This type of reaction is called endothermic (literally, inside heat, or absorbing heat).