The Hephaestus es is the movil base for the Colonial Strategic Homeland Intervention, Enforcement, and Logistics Division.
The objetive of SHIELD is protect humanity from the alien treat, but using spionage and electronic surveillance, is more subtle than the military arm called NEST.
The Shield have a movil base, that is a modificated Gurdian Base Star captured during the first Cylon War, the base is heavy armed, and have all types of technological devices to trace any alien in any Colony, in the Cyrannus System, they can acess to any data, email, camera, cell phone, in the colonys when they get in their orbit.
Shields is a higher division than the Colonial NEST, but their objetives are similar, but at difference from NEST, that is a pure military organization, SHIELD have military and civilian working together.
And they work is more secret and less known, not even the NEST have known about SHIELD.
Other objetive of SHIELD is protect the Colonials from any kind of Cylon treat.
All parts from CanisD.