Functional group is a determining factor in chemistry that determines the properties of a chemical compound. Just for example, OH group determines that the organic compound is alcoholic in nature. CO group determines that the organic compound is ether. All of the organic compound containing CO group are in same properties. COOH group is the acid group. It is called Carboxil group. Any organic compound containing COOH is Carboxilic acid. They shows acid properties. Different compound with same functional group shows same properties. There chemical reaction is same. In a few factor they may shows different properties.
The examples of few functional group is
Methyl group, Ethyl group, Ethylene group, Carboxil group, Aldehyde group, Ketone group, Ester group etc.