One of the goals for Active Directory was to get away from the 'only one writeable copy' Primary Domain Controller concept we had in NT, and make it so that AD changes could be made on any DC and they would get replicated around. A great idea, except there are a couple of things that really do need to have a single instance, to avoid data corruption or duplication. These roles are called FSMO's :Flexible Single Master Operations. You can read more about what they actually are and what they do here. When you first run dcpromo.exe to build your domain, all 5 FSMO roles will be assigned to the first DC that you install. For a small domain, that's fine, but there are situations where it makes sense, and may even be necessary, to move some or all of the roles. (more)
One of the goals for Active Directory was to get away from the 'only one writeable copy' Primary Domain Controller concept we had in NT, and make it so that AD changes could be made on any DC and they would get replicated around. A great idea, except there are a couple of things that really do need to have a single instance, to avoid data corruption or duplication. These roles are called FSMO's :Flexible Single Master Operations. You can read more about what they actually are and what they do here. When you first run dcpromo.exe to build your domain, all 5 FSMO roles will be assigned to the first DC that you install. For a small domain, that's fine, but there are situations where it makes sense, and may even be necessary, to move some or all of the roles. (more)
FSMO Placement Overview