An aromatic ring is a hydrocarbon, such as benzene, that has a low hydrogen to carbon ratio, and has low chemical reactivity due to aromatic resonance. In the case of benzene (formula C6H6), the carbon atoms form a hexagonal ring, with each carbon atom bonded in series at each point of the hexagon. Because of orbital hybridization, each carbon atom can bond to only one hydrogen. Although the traditional illustration of an aromatic ring appears to contain double bonds, in reality the extra pi electrons are equally distributed around the ring because of resonance. This results in an exceptionally stable molecule which does not undergo markovnikovian addition reactions.
There are other shapes as well, but in order to be aromatic the ring must satisfy Huckel's rule by having a number of free electrons which satisfies the equation 4N+2. Cyclooctotetraene is an example of a molecule which does not follow this rule. Although it is a ring composed of alternating double bonds, it's chemical reactivity is that of a non-aromatic alkene.
There are variations to the benzene molecule. One such variation is toluene (formula C7H8, also called methylbenzene) where a methyl group replaces a hydrogen atom. Other variations can have non-carbon substituents in the ring whose free electron pairs contribute to the de-localized aromatic field and help it satisfy Huckel's rule.
Aromatic rings can also form in groups or chains, where each ring is "joined at the hip." These are called polycyclic benzenoid aromatic hydrocarbons. One such case is naphthalene (formula C10H8), where two benzene rings share one carbon bond. These compounds must also follow Huckel's rule in order to be aromatic.
Aromatic rings were called aromatic in the early days of their discovery because they are fragrant, as they were obtained from resins or oils.