What is a functional group? Give examples of four different functional groups.

(N/A) functional group is an atom or a group of atoms joined in a specific manner that is responsible for the characteristic chemical properties of organic compounds.
Examples of four different functional groups are:
$1$. Hydroxyl group: $-OH$
$2$. Aldehyde group: $-CHO$
$3$. Carboxylic acid group: $-COOH$
$4$. Ketone group: $>C=O$