FSMO roles are unique within a single domain. two DCs from two different domains can have the same role. forest wide roles are completely unique across the whole forest
a DC holding an FSMO role is known as an Operations Master
once the operations master processes a change. the update is replicated to all other domain controllers in the forest or domain via SYSVOL Replication
there are 5 FSMO roles that are divided into two categories:
forest wide roles
domain wide roles
forest wide roles
Schema Master
updates the AD schema that defines the objects and their attributes
Domain Naming Master
manages the addition and removal of domains in the forest
domain wide roles
Relative ID (RID) Master
allocates pools of unique Security Identifiers (SID)s to DCs to generate SIDs for all objects