In the context of organic molecules, aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, be it phenyl, naphthyl, thienyl, indolyl, etc. (see IUPAC nomenclature). "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used as a placeholder for the aryl group in chemical structure diagrams.
A simple aryl group is phenyl, C6H5; it is derived from benzene. The tolyl group, CH3C6H4, is derived from toluene (methylbenzene). The xylyl group, (CH3)2C6H3, is derived from xylene (dimethylbenzene), while the naphthyl group, C10H7, is derived from naphthalene.
Arylation is a chemical process in which an aryl group is attached to a substituent.