A functional group is any one of a number of specific combinations of molecules within an atom. Used mainly in organic chemistry, functional groups are a helpful way to determine the structure and function of a molecule, as functional groups act very similar no matter what molecule they are a part of .
A generic functional group is represented by the letter R in a chemical formula. For instance, CR4 represents a carbon with four of the same functional group bonded to it.
Some functional groups are described in the following table: 
|Alkane||Carbon-carbon single bond||-ane||Ethane (CH3CH3)|
|Alkene||Carbon-carbon double bond||-ene||Ethene (CH2CH2)|
|Alkyne||Carbon-carbon triple bond||-yne||Ethyne (CHCH)|
|Arene||Unsaturated six-carbon ring||none||Benzene (C6H6)|
|Halide||covalantly bonded F, Cl, Br or I||none||Chloroethane (ClCH2CH3|
|Alcohol||Covalantly bonded -OH||-ol||Ethanol (CH3CH2OH)|
|Ketone||Double bonded O||-one||Methyl ethyl ketone (CH3(CO)C2H5)|
|Aldehyde||Double bonded O attached to a carbon attached to a hydrogen||-al||Acetaldehyde (C2H4O)|
|Ether||Oxygen bonded to two other carbons||ether||Diethyl ether (CH3CH2OCH2CH3|
|Ester||Oxygen double bonded to a carbon which is also bonded to another oxygen||ester||Diethyl ester (CH3CH2OOCHCH3|
|Amide||Double bonded O attached to a carbon attached to a nitrogen||-amide||Ethanamide (CH3CONH2)|
|Carboxylic acid||Carbon double bonded to O and bonded to -OH||-oic acid||Ethanoic acid (CH3COOH)|
- McMurry, John. Organic Chemistry, 6e. Brooks/Cole: 2004.