Homologous series is a family of organic compounds with related properties.
The members of this series look alike due to common atomic groups, they differ due to an increasing number of CH2 groups.

Example: Homologous series of alkanols (R-OH)

                    Methanol has a hydrocarbon rest  CH3-,
                    ethanol has a hydrocarbon rest     CH3-CH2-,
                    propanol has a hydrocarbon rest  CH3-CH2-CH2-,
                    butanol has a hydrocarbon rest    CH3-CH2-,CH2-CH2-