A docking bay was an area where starships could offload or receive personnel or cargo.
Docking bays came in various sizes depending where they were located but all had a few commonalities. All docking bays were located at the outer edge of a space station or starbase where doors protected them from space. The docking bay could be entered via an airlock.
A docking bay could contain anything from a shuttlecraft to multiple Galaxy- or Excelsior-class starships. An example of a docking bay was that of the Federation starbase Earth Spacedock. Spacedock, along with Starbase 74, had one of the largest docking bays in the Federation. (Star Trek III: The Search for Spock; TNG: "11001001")
Not all docking bays were located within a structure. Deep Space 9 had a total of twelve docking bays of which six were external, called docking pylons. They were used to facilitate large ships which did not fit into the internal docking bays. (DS9: "Visionary")