These are a lot of questions to answer in a single response.
A domain controller is a server role that is responsible for authentication and authorizatoin services in a domain. What is a domain? A domain is a centralized system that stores the security principles for your network resources (user, computer, group accounts, etc..).
A domain controller is generally implemented once you have 10 or more computers on the network, where a simple workgroup is too difficult to manage with repect to securing your accounts, resources, services, etc..
I have several dozen articles that cover Windows Server 2008 roles including DC, DNS, DHCP, etc... Feel free to take a look and come back with questions. http://www.itgeared.com/topic-2/windows-server2008/
I would recommend that you create a thread for each specific question. It will be too difficult to maintain a thread with more than one question.
JorgeM
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