The aether is the substance that exists between planets and other bodies in outerspace. It is the medium through which light and very small amounts of gravity flow. Like a great ocean, aetherspace has endless eddies and currents. Most notably, currents run to and from stars to planet sized bodies that orbit those stars. Stars also produce a solar wind that blows in all directions, not just along the paths of the aetheral currents.
Any aethership takes advantage of these currents as much as possible. Sails are also in use for tactical speeds and maneuvering. The main propulsion is provided by a vestibule, so named because it is a bridge between the world and the space between where all aetherships go. (a sort-of hyperspace) It is a specially crafted room on all ships.