A functional group is a specific group of atoms within a molecule that is responsible for the characteristic chemical reactions of that molecule. These groups determine the properties and reactivity of organic compounds. Common examples of functional groups include hydroxyl, carboxyl, and amino groups, each imparting unique features to the molecules they are part of. Functional groups are essential in organic chemistry because they help classify compounds and predict their behavior in chemical reactions. By identifying the functional groups present, chemists can infer how a substance will interact with other chemicals, making them crucial for understanding biological processes and designing new pharmaceuticals.