Chemical reactions that release energy in the form of heat, light, or sound are called exothermic reactions.

Example : Mixture of sodium and chlorine to yield table salt

$Na _{(s)}+\frac{1}{2} Cl _{2(s)} \longrightarrow NaCl _{( s )}+411 \,kJ$ of energy

In other words, combination reactions are exothermic.

Reactions that absorb energy or require energy in order to proceed are called endothermic reactions.

For example: In the process of photosynthesis, plants use the energy from the sun to convert carbon dioxide and water to glucose and oxygen.

$6C{{O}_{2(g)}}+6{{H}_{2}}{{O}_{(l)}}\text{ }\xrightarrow{\text{Sunlight}}\text{ }\underset{Glucose}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6(aq)}}}}\,+6{{O}_{2(g)}}$