Final answer:
Polycyclic aromatic hydrocarbons (PAHs) are compounds with fused ring systems, while heterocyclic compounds have rings with non-carbon atoms like nitrogen.
Step-by-step explanation:
Compounds that contain a fused ring system are known as polycyclic aromatic hydrocarbons (PAHs). These structures consist of multiple aromatic rings, such as benzene rings, that are joined together and share one or more sides or edges. For instance, the basic structure of steroid hormones includes a system of three six-membered rings and one five-membered ring that are connected in this way. These rings have bridgehead carbons that link them together. Furthermore, some cyclic compounds contain atoms other than carbon, called heterocyclic compounds. Notable examples are purine and pyrimidine, which are key components in nucleic acids and contain nitrogen atoms within their fused ring structures.