Saturated and Unsaturated Hydrocarbons

Organic compounds containing atoms joined by only single bonds are saturated compounds.

For example, all compounds belonging to the alkane homologous series are saturated compounds. Saturated compounds do not undergo addition reactions. For example,

Unsaturated compounds are organic compounds in which there is at least a pair of carbon atoms which are joined by double bond or triple covalent bonds. e.g. Alkenes (contain double bonds) and Alkynes (contain triple bonds) are unsaturated. Unsaturated compounds will undergo Addition reactions.

Functional Group:

A functional group is an atom, group of atoms or bonds which determines the chemical properties of an organic compound. All compounds with the same functional group will undergo similar chemical reactions although they may have different physical properties. 

Some homologous series with their functional group are given in the table below: