The global catalog server acts as a central repository for Active Directory by holding a complete copy of all objects within its local domain and a partial copy of all objects from other domains within the same forest. It facilitates searches, resolves UPN names, and provides universal group membership information. Flexible Single Master Operations (FSMO) roles assign single-master operations like schema updates, domain naming, and relative identifier assignment to specific domain controllers to prevent conflicts. The document discusses best practices for global catalog placement, universal group caching, viewing and transferring FSMO roles, and seizing roles during failures.