A Space Station is a spacecraft capable of supporting crew, designed to remain in space for an extended period of time. In Space Agency, it can also be designed to travel. While in orbit, its classified as an artificial satellite.
See this page for examples of different types of Space Stations.
You can expand or replace the space station if you want to after creating a sandbox. The structure of the station will differ in accordance to the level that you were on Career Mode at the time of when you had made your sandbox.