Air routes differ over airways in that they are in uncontrolled airspace. That is, air routes are not controlled by ATC, unlike airways.
The Designated Airspace Handbook describes them in detail:
Low Level Airway: Controlled low level airspace, extending upwards from 2200 feet above the surface of the earth within the following specified boundaries […]
Low Level Air route: Class G Airspace, extending upwards form the surface of the earth, within the following specified boundaries […]
Low level air routes are usually designated with the prefix: AR, BR, and RR. Because they are mostly in uncontrolled airspace in the northern parts of the country. The majority of air routes are between two LF/MF navaids (NDBs).
Low level airways are usually designated with the prefix: A, B, R, and V.
RNAV Routes are designated with the prefix T for controlled and L for uncontrolled RNAV Routes.
