Q.What is meant by a functional group? Identify the functional group in the following compounds.

With millions of known organic compounds, classifying them by two independent lenses -- overall carbon-skeleton structure, and functional group -- is what makes the subject navigable at all.

By structure, every compound is first split into acyclic (open-chain/aliphatic, like propane $CH_3-CH_2-CH_3$, no ring anywhere) or cyclic (containing at least one ring). Cyclic compounds split again by ring composition: homocyclic (the ring is built entirely of carbon) versus heterocyclic (the ring contains at least one non-carbon atom, commonly O, N or S); and each of those splits again by aromaticity. So a homocyclic ring is either alicyclic/carbocyclic (a plain, non-aromatic carbon ring, e.g. cyclopropane) or aromatic -- itself either benzenoid (built around an actual benzene ring, e.g. phenol) or non-benzenoid (aromatic without a benzene ring, e.g. azulene). A heterocyclic ring is either alicyclic (a non-aromatic heteroatom ring, e.g. tetrahydrofuran) or aromatic (e.g. pyridine). Applying the scheme in practice just means asking, in order: is there a ring at all? if so, does it contain a heteroatom? and is it aromatic? …