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.  A good overview on FSMO placement is here: http://support.microsoft.com/kb/223346/EN-US/
If you lose a FSMO DC for a little while, you'll probably be ok.  In fact, product support recommends that whenever possible, if you can get that DC back up and running you should do so.  If the server died and is staying dead, and you didn't get a chance to elegantly remove it from the domain and reassign the FSMO roles, you can use ntdsutil.exe to seize the role, forcing another DC to take over for your dead server.
So what happens if I successfully sieze the role, and then, miraculously, I recover my dead server and put it back on the network?  It still thinks it owns the FSMO role, and doesn't know it was seized by another server.  Now I have 2 FSMO's.  What happens now?  In the old days, bad things would happen.  Data corruption and other kinds of hysteria.
We live in better days now.  If you're running Windows Server 2003, or Windows 2000 Server SP3, when the old server comes back online, it doesn't do any FSMO stuff until it has successfully completed replication.  One of the AD attributes that is replicated is the FSMO role owner.  If the server sees that the attribute has changed to a different server name, it quietly demotes itself from being a FSMO server and carries on.  If it sees that it is still listed in the attribute as the FSMO owner, it will then begin doing its FSMO roles.  You can read more about it here: http://support.microsoft.com/default.aspx?scid=kb;en-us;305476