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Aromatic hydrocarbons are a class of organic compounds that have a conjugated cyclic structure with alternating single and double bonds. They exhibit specific criteria of aromaticity, which include resonance, planarity, and a specific number of electrons in their conjugated system.

Resonance is the phenomenon where a molecule can be represented by multiple Lewis structures or resonance forms. These forms represent the same molecule, with the same atoms but different arrangements of electrons. In reality, the electrons are distributed evenly throughout the molecule inducing stability. Aromatic hydrocarbons have a high degree of resonance stabilization.

All aromatic hydrocarbons share a common structure, which is the presence of a conjugated cyclic ring, where each carbon atom has a planar geometry (sp2 hybridization). The most common and simplest example is benzene, a six-membered ring with alternating single and double bonds. Benzene has six pi electrons, so it satisfies the criteria for aromaticity known as Hückel's rule. In general, aromatic hydrocarbons follow the Hückel's rule which states that the total number of pi electrons in their conjugated system should be equal to (4n + 2), where n is a whole number (0, 1, 2, ...). This leads to the prevalence of delocalized pi-electron systems in aromatic hydrocarbons, which plays a significant role in their chemical properties. In summary, the common structure of all aromatic hydrocarbons is a planar, conjugated cyclic ring system that follows Hückel's rule and displays resonance stabilization.