What are functional groups?

Hint: Organic compounds are generally hydrocarbons having carbon and hydrogen. These compounds, apart from carbon and hydrogen contain some other chemical units, which completely alters their chemical composition. These are called functional groups.

Complete answer:
Functional groups are the extra chemical species or molecules present in any organic compound. Functional groups can be substituted on the hydrocarbon chain in the hydrocarbons. The addition of the functional groups, not only changes the name of the organic compound, but also gives it a change in the chemical properties, and the ability to react with certain reagents.
The functional groups are responsible for the activity of the organic molecule in various reactions. The functional groups react in the same manner irrespective of the nature or length of the hydrocarbon chain.
Some common functional groups are,
$\bullet$ Alcohols with the formula, OH
$\bullet$ Aldehydes with the formula, CHO
$\bullet$ Ketones with the formula CO, have a double bond between carbon and oxygen and are called carbonyl and added in the middle of the carbon chain.
$\bullet$ Carboxylic acids have a formula, COOH. They are a class of acids, and give acidic character to the molecule.
$\bullet$ Esters with the formula RCOOR, where R is any alkyl group, same or different.
$\bullet$ Ethers with formula, RCOR, where R is any alkyl group, maybe same or different.
$\bullet$ Halides that contain halogens, F, Cl, Br, etc.
Hence, a functional group is any compound that gives a unique character to the organic molecule.

Note:
Compounds have changed in their name after adding functional groups, like for alcohols with alkanes named as ‘alkanol’ , for aldehyde, ‘alkanal’, for ketone ‘alkanone’. For carboxylic acid, ‘alkanoic acid’ is added. For halides, haloalkanes is written. For ether, alkyl ethers, for esters, alkyl alkanoates are used.