SR2020 "Bases" are military facilities that provide a certain level of defense from attack, a repair facility and a repository for housing many military units of the Base's class in reserve.
There are three types of military Bases - the Barracks, Air Field and Sea Pier, which represent an Army base, an Air Force base and a Navy base respectively.
The above mentioned Bases can be build around civilian settlements such as cities, towns and villages. However each Military Complex with a Military Fabrication facility node such as an Aircraft Fabrication must also possess a corresponding reserve repository/base node. This reserve base houses the un-deployed units produced from the "Fab".
All three Base-types serve as repair facilities for their own type of unit. Damaged aircraft sent to Reserve or sent for Repair inside an Air Field Base will slowly be repaired. If the aircraft was sent to the Base for Repair then once its repairs are completed, the aircraft will automatically be re-deployed onto the Map. Otherwise it will stay in a non-deployed, Reserve status. The same holds true for land and sea units and their corresponding Barracks and Sea Piers.
SR2020 Bases are distinct from military fortifications such as Fortifications, Radar Stations and AA defenses in that such fortifications cannot house reserved units, cannot perform repairs and do not affiliate with "fabs".
Air Fields and Sea Piers are generators of Supply which can enrich the Map hexes that surround the Base. Barracks, on the other hand, provide no such source of Supply.
Naval vessels cannot be placed upon Sea Pier hexes, since technically these piers are on land. Therefore sea-piers provide no defensive bonus for nearby deployed naval vessels.