AD uses a multi-master replication mechanism, meaning that updates can originate on any RWDC. For all kinds of services AD is highly redundant assuming you have more than one RWDC. Within AD some operations cannot operate using the multi-master principle, but rather use the single-master principle to ensure consistency. The roles for those operations are the so called Flexible Single Masters of Operations (FSMO). From a forest perspective two forest wide FSMO roles exist and from a domain perspective three domain wide FSMO roles exist. Below you will find which one is which.
Complete article : blogs.dirteam.com/blogs/jorge
