I wanted to know the use of having multiple domain controllers for one domain And some more questions related to it are:
If there are multiple domain controllers can computer information in one domain controller visible to others? If there are two computers having a saame domain name shown as xyz in control panel is there a possibility of them to be in a different domain?
PS:I am new to AD and could not find straight forward answer.
The primary reason for having multiple domain controllers is for fault tolerance. They will replicate the Active Directory information between them and can provide services if the other is unavailable.
Having multiple DC's is a best practice standard. You should always have at least 2 DC's to safe guard against one going down. If all you had was one, and it went down, there'd be nothing for your objects to authenticate to.
It's very common for environments to have more than 2 DC's as well. There are a lot of factors which go into deciding if a new DC is needed. For example, you may have more than one physical location and want to have DC's on each site. When DC's enroll into an existing domain, they're able to find each other and share information.
You can also have multiple forests (groups of domains) linked together. You can do this by attaching them to the same "site" (using a directory manager like IDM) or by establishing trusts between the separately configured domains. However, most needs are satisfied by simply creating a new OU in an existing forest. You'll usually see multiple domains configured together as part of mergers and migrations or because someone had an older Novell IDM infrastructure.
Given the freedom which Virtual Machines offer, I'd highly recommend you get something like virtualbox and download a release preview of Microsoft Server 2012 R2 (currently free) and build your own domain. Feel free to play around and readh through Microsofts online tech databases.
http://technet.microsoft.com/en-us/library/cc786438(v=ws.10).aspx
