Hydrocarbons are organic molecules composed only of carbon and hydrogen. Aromatic molecules, originally named for their sensory distinction, now refer to a special bonding situation with a cyclic array of pi orbitals and electrons.

The aliphatic group refers to the remainder of the hydrocarbons and includes those with only C-C or C-H single bonds (i.e., alkanes and cycloalkanes), a double bond (alkenes), or a triple bond (alkynes).