Server?? what does it do and what is required?

A server is a computer whose purpose is to "serve" files or tasks. A web server hosts a website and all of the files within. Whereas a file server (e.g. a ftp server), serves files to people. As to your question about the difference between the Registry, Registrar, and Registrant, I am at a loss to what you are asking. But the registry on windows is where windows keeps all of it's system files.