Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. They determine the properties and reactivity of organic compounds, influencing how they interact with other substances. Common functional groups include hydroxyl, carboxyl, and amino groups, each imparting unique features to the molecules they are part of. These groups are essential in organic chemistry, as they help classify compounds and predict their behavior in chemical reactions. Understanding functional groups allows chemists to design and synthesize new compounds with desired properties, making them crucial in fields like pharmaceuticals and materials science.