R-CH3 is an organic composite obtained starting from methane (CH4) and substituting one of the hydrogen atoms with an organic radical, generally indicated with R. Frequently the functional group -CH3 is also called methil group.
Important examples are acetic acid, where R is the acid group COOH, so that the acetic acid has molecular formula CH3COOH and the methanamine, where the functional group is the amine group (NH2) so that the molecular formula results to be CH3NH2.