Heterocyclic compounds are organic molecules that contain a ring structure made up of at least one atom that is not carbon. These non-carbon atoms, often nitrogen, oxygen, or sulfur, are called heteroatoms. Heterocyclic compounds are important in many fields, including pharmaceuticals, as they form the backbone of many drugs and biological molecules. These compounds can be classified into two main categories: aromatic and non-aromatic. Aromatic heterocycles, like pyridine and furan, have special stability due to their electron-rich structure. Non-aromatic heterocycles, such as pyrrolidine and thiophene, do not have this stability but still play crucial roles in various chemical reactions and applications.