Aromatic hydrocarbons are organic molecules containing one or more benzene rings. These have alternating double and single bonds in a 6 carbon ring. This makes the electrons in the double bonds delocalized. Sort of like, on average each carbon has two one and half bonds with its neighbors. This makes the ring very stable and means that carbon can only have one additional bond. This means a benzene ring has 6 possible reaction sites for a single bond. Just about anything that can make a single bond can replace one of those hydrogens.
Benzene rings can also be attached to another at a common side, such as napthalaene.
Bigger rings with even numbered carbons are possible, but much less stable.
