Root server is not a single server but a group of servers, I think 13 but not sure, at the top of internet name space. It means there a group of servers, to load balance, which contains same information and are referred as root or ".". For more details plz refer:

http://www.howstuffworks.com/dns.htm
http://technet2.microsoft.com/Window...7fb7c1033.mspx