Animal tissues, plant tissues, bacteria, and fungi contain organic molecules; horns and nails, fallen leaves, eggs, fruits and vegetables contain organic compounds; wood, milk, paper, petroleum and gasoline contain organic compounds. In summary, all living matter, parts or products of living matter and remains of living matter contain organic compounds. Organic molecules associated with living organisms are also called biomolecules.

Organic compounds are molecules that contain carbon atoms covalently bonded to hydrogen atoms (C-H bonds). Many organic compounds are formed from chains of covalently-linked carbon atoms with hydrogen atoms attached to the chain (a hydrocarbon backbone). This means that all organic compounds have in common the presence of carbon atoms and hydrogen atoms. In addition, different organic compounds may contain oxygen, nitrogen, phosphorous, and other elements. Carbon dioxide (CO2) does not have hydrogen; then, it is not an organic compound. Water (H2O) has no carbon; then, it is not an organic compound. Sodium chloride has neither carbon nor hydrogen; then, it is not an organic compound. Generally, gases, and mineral salts (inorganic substances found in soil, or bodies of water or watercourses) are not organic.