The UNSC Security Council is the governing body for the UNSC. As an emergency military government, the Security Council is mostly made up of military personnel, tasked with governing Earth and other UNSC colonies and coordinating the operations of the UNSCDF. It seems to have superseded the Unified Earth Government, formed after the Interplanetary War, as Humanity's overall governing body. However, it is apparently beneath the UNSC Security Committee. The fact that the Security Council is guarded by UNSC Marines suggests that it is mostly centered around naval operations.
The Security Council is apparently based in Sydney, Australia, deep underneath UNSC HIGHCOM Facility Bravo-6. It was mentioned during a meeting of the UNSC Security Committee, presumed to be an independent administrative military branch.
The UNSC Security Council is likely to be similar to the UN Security Council prior to the formation of the UEG. The fact that James Ackerson, a Colonel in the UNSC Army, is a member indicates that military personnel may serve on it as well.