implementing custom domains
Just wondering how sites like tumblr or weebly are able to implement custom domains. For example when you first sign up you get a sub domain like "user01.tumblr.com". But they allow you the option of changing that to your own domain, provided that you supply the nameservers.
I just want to know what is going on behind the scenes...
(PS: sorry for a very basic question, I am fairly new to server administration and still trying to get my head around how DNS works).
Any help would be greatly appreciated. Many thanks!