Doors are orange portals generated by The Carrier that allow instant movement from one place to another. The Carrier exists outside of space and time, but somehow also in the same place as Earth. Since the Carrier is touching all of Earth at the same time, stepping through a Door can take you anywhere on (or around) the planet. Though a Door can only be opened to a place around Earth (the furthest distance a Door has been opened to is the sun), Doors can open to places in Earth-space in other dimensions, allowing for travel all across the multiverse. The Carrier can also open Doors in different times.
All members of the Authority are able to telepathically summon Doors (though usually the command "Door" is spoken aloud) that can take them anywhere they want.