Supreme Ruler has different military bases used to train, supply, and maintain combat units. There are separate bases for air, land, and sea units. Besides that, bases differ mostly in their build capacity and how large of an effect they have on supply. With supply, note that air bases do not provide supply, and obviously ground and sea bases work only on their own kind of terrain. Also, bases have almost no spotting ability by themselves. They need nearby radar stations or units to spot other players' units.
Maintenance figures may not be exact.
Airstrips are paved runways with little permanent military infrastructure and no hardened aircraft shelters. As such, aircraft attacked while based on one will be less protected than at an air base. This base has no unit production or reserve ability.
Airbases are fully-functional military aerodomes, with production facilities and aircraft shelters. Aircraft squadrons are produced here, three at a time, and can also be held in reserve. This the only upgrade used to build missiles! Each airbase can construct six missles at once. And remember, airbases themselves have almost no spotting ability.
A small outpost that produces only one unit.
The small military base produces three units at a time. It's the smallest military base that triggers close combat when defended. The developers refer to small, medium, and large military bases as one, two, and three star bases.
The medium military base produces six units and offers some economy of scale advantages over two small military bases. It also takes longer to build than a single small military base. A medium military base can give a good boost to supply. Make sure they're hooked up to your road and rail infrastructure, and think about putting one near industries to boost output (if you don't mind making an attractive target).
Large military bases produce the most units with the most economy of scale savings. Its supply effect is obviously the greatest also. Note that the larger the military base, the more reserve units it can hold.
Sea piers are minor bases with no ability to produce units or project supply. They can be used to refuel and rearm units, but will have no effect on nearby hex's supply.
This base is the only that may produce naval units and project supply on ocean terrain. Six units of any size may be under construction at once, and reserve ships are held here. Because of its supply effects, a port is a prerequisite for building an oil platform.