Sorry, I didn't realise it was a stupid question. For those who don't think its a stupid question, I'll try to elaborate.
John Doe signs up for hosting and a new domain. The domain gets registered through the registrar I'm using at the time. Twelve months down the track I decide to start using a registrar with better service and prices. Now John Doe's domain is coming up for renewal. How would I transfer John's domain from the old registrar I was using to the new one I'm now using. Keep in mind that John doesn't really need to know that the domain is being moved to anther registrar, because as far as he is concerned the domain is just part of the whole hosting package.