Survivors are those who survive the invasions of hell. In most cases the only known survivor is Doom Marine (or other playable Marines or characters).
Doom 1-2, Final Doom, Doom 64
In these games there are no known survivors, other than Doomguy himself.
Doom 3, Resurrection of Evil, & Lost Mission
In this game its strongly suggested that the Marine Transfer is the only survivor. There are at least survivors of the initial invasion throughout the game.
Lost Mission confirms there are at least a few other survivors. Including some on board DarkStar (one appearing to be Marcus Caseon) transport ship that has returned to land on Mars to pick up survivors.
The Marine Engineer from Resurrection of Evil also survives, and Elizabeth McNeil. The intro suggests only one survivor was located during earlier events of Doom 3.
Doom 3 novels
Doom RPG/Doom II RPG
The Doom Marine and a few others survive in the post ending return to the game (however respawning enemies might suggest that the incursion was still ongoing).
However, Doom II RPG comic suggests rumors have spread that there are no known survivors (however its unclear in that game if it was referring to events of Doom RPG or Doom 3).
Both Doom Marine and Samuel Hayden are still alive at the end.