Functional group is an atom or group of atoms within a molecule that determine the properties of the molecule and reactions in which it takes part.
Functional group present in alcohols, aldehydes, ketones and carboxylic acids are:
Alcohols : hydroxyl group (OH)
Aldehydes : aldehydic group (CHO)
Ketones : ketonic group (C=O)
Carboxylic acid : carboxyl group (COOH)