A saturated, unsubstituted hydrocarbon has a formula of C_nH_(2n+2). Alkanes are said to be "saturated". Try this out with methane, ethane, propane, etc. Each double bond, or ring junction, reduces the hydrogen count by 2, and so introduces "1 degree of unsaturation". "Cyclohexane" or "hexene" thus have formulae of C_6H_12.
Aromatic compounds generally feature the benzene ring, which imposes "4 degrees of unsaturation"; i.e. consider the benzene molecule, C_6H_6, where hexane would by C_6H_14. The extra electron density on the aromatic ring (the six delocalized pi electrons) offers peculiar chemisty; distinct from the chemistry of the alkanes.