What are R groups in organic chemistry?

R group in Organic chemistry refers to the alkyl group which may be either straight chain ( like -CH3, -C2H5,etc) or branched [ isopropyl -CH(CH3)2 or neobutyl -C(CH3)3,etc]. This alkyl group is due to the substitution of a hydrogen atom of alkane ( from alkane to alkyl) with any other functional group. 
Even aromatic compounds do contain alkyl group. Just a simple example if we remove a hydrogen atom from benzene (C6H6) we get a phenyl group which is an alkyl group (-C6H5) therefore in chlorobenzene, phenyl is the R part.

For example if you look at this reaction:

only the top left OH group is reacting so a simplified version would be

for example if you look at this reaction

only the top left OH group is reacting so a simplified version would be