Heterocycles are cyclic compounds that contain at least one atom in the ring that is not a carbon atom. These non-carbon atoms can include elements like nitrogen, oxygen, or sulfur, which contribute to the unique chemical properties of the compound. Heterocycles are important in various fields, including pharmaceuticals, as many drugs are based on heterocyclic structures. Common examples of heterocycles include pyridine, furan, and pyrimidine. These compounds are found in many natural products and play crucial roles in biological processes. Their diverse structures allow for a wide range of applications in organic chemistry and materials science.