Organic molecules

All organic molecules contain carbon; this is due to its chemical properties that allow it to produce the individuality of living things.
A Carbon atom has four electrons in its outer shell, and becomes stable when it loses or gains four electrons.
This would be too many to lose or gain, as the charge when losing and gaining four electrons would be either 4⁺ or 4^-, which is too highly charged. So carbon forms covalent bonds.
For example, in methane a carbon atom bonds with four hydrogen atoms, sharing their electrons.
Carbon can form a strong covalent bond with itself (catenation) and so can form long chains. This leads to a limitless variety of carbon compounds.

Hydrocarbons

Hydrocarbons are compounds containing Hydrogen and Carbon only.
There three different categories of Hydrocarbon: alkanes, alkenes and aromatic hydrocarbons.
Aromatic hydrocarbons are derived from benzene.
Aliphatic hydrocarbons have an open chain structure.