A functional group comprises slightly different configurations of the same organic compound, all with similar behaviors. A functional group is typically a molecular subset of a larger molecule. The same functional group produces selfsame or similar reactions regardless of the molecule of which it is part.
An organic compound’s functional group is indicative of the entire molecule’s reactive properties. Reactions of an organic compound can be predicted by knowing the kind(s) of functional group(s) it has.
Carbon molecules are commonly classified based upon their functional group. Proteins, the building blocks of life, are complex carbon-based macromolecules comprising many functional groups. In doing their job, proteins selectively employ a certain functional group as a tool.
The term moiety is often used as a synonym for functional group, but a moiety is a specific segment of a molecule, regardless of functional group. A moiety may be part of a functional group or encompass parts of different functional groups.