The Space Security Service or "SSS" was a military force circa 4000. (TV: The Daleks' Master Plan) Its purpose was to gather intelligence on behalf of the Solar System and when necessary, eliminate threats to that safety. They considered the Daleks paramount amongst these threats, and it wasn't uncommon for SSS agents to find themselves in battle against them.
According to one account, the SSS had its headquarters in Central City on Earth, where computers held records of the chemical makeup of every person in the Solar System. (TV: The Daleks' Master Plan) According to another, this may have been a lie. Instead, the SSS base was actually inside an unknown mountain on an unknown world, with contact through a more public location on each member world. (PROSE: The Outlaw Planet) Certainly, one publicly-known "branch location" was in Carrington City on Micawber's World. (PROSE: Placebo Effect)
Agents wore uniforms and carried blasters. (TV: Mission to the Unknown) The Service had ranks. First Rank was achieved some time after an agent joined the service and Second Rank had greater responsibility. (TV: The Daleks' Master Plan) One agent, Marc Cory, carried two documents, one giving him license to kill, the other the power to enlist any person to aid him in his mission. This included civilians. (TV: Mission to the Unknown)
Agents could not afford to have sentiment or compassion. Sara Kingdom, a Space Security agent, killed her own brother, Bret Vyon, also an agent, on orders. Though Kingdom got her orders from Mavic Chen, the Guardian of the Solar System, the Space Security considered no one above suspicion, including Chen himself. (TV: The Daleks' Master Plan) Serving the SSS was a lifelong commitment, with each agent dedicating all of their time to the defending the Solar System. For this reason, female members of the SSS are required to undergo a procedure which renders them unable to bear children. (AUDIO: An Ordinary Life)