The responsibilities of Domain Controllers in Active Directory have been divided into five different roles. These roles are known as Flexible Single Master Operation (FSMO) roles. These five roles together make a complete Active Directory system. The roles are:
Schema Master - one per forest
Domain Naming Master - one per forest
Relative ID (RID) Master - one per domain
Primary Domain Controller (PDC) Emulator - one per domain
Infrastructure Master - one per domain