Active Directory uses the multi-master model, where domain controllers are all able to respond to client requests. When comparing to the old Windows NT 4 server-style Primary Domain Controller (PDC) and Backup Domain Controller (BDC) models, where BDCs were read-only until switched to become the only PDC, this model offers many benefits.
However, some tasks within Active Directory don't work well with the multi-master model. When designing Active Directory, Microsoft created five roles outside of the model, labeled FSMO roles, to prevent conflicting updates.
These roles can be flexibly assigned to domain controllers and seized when a domain controller holding the role fails. These roles offer single master operations within the scope of the roles. Hence the name FSMO role.
In every Active Directory environment there are five roles with two different scopes...