This answer will explain what propagation is, and how it will affect your domain after it is registered or transferred between providers.
'DNS propagation' is a term to describe the updating of information across the Internet. It takes two forms: changes to your DNS Zone and changes to your WHOIS information.
A change to your DNS Zone information typically affects only a handful of servers, and completes much more quickly. However, the WHOIS is the master record which tells every DNS server in the world the server for your domain. A change to your WHOIS information (such as updating the ADMIN-C in your control panel) can take up to 72 hours to propagate fully.
Customers often call 123-support to say that they can't see their domain names straight after ordering them, or just after a transfer between providers. After either of these two events your domain name/website may not be visible for up to 48 hours, mainly because millions of DNS servers need to be updated with the new information.