A homologous series is a series of carbon compounds that have different numbers of carbon atoms but contain the same functional group.

For example, methane, ethane, propane, butane, etc. are all part of the alkane homologous series. The general formula of this series is $C_nH_{2n+2}$

Methane $CH_4 $

Ethane $CH_3CH_3$

Propane $CH_3CH_2CH_3$

Butane $CH_3CH_2CH_2CH_3$

It can be noticed that there is a difference of $-CH_2$ unit between each successive compound.