Domain Propagation is the process of your domain's information, like the server IP which your site is hosted on, being sent to all the other name servers in the world. Obviously because there are a lot of name servers in the world, this causes a delay, the specific delay depends on a lot, each server refreshes their cache of this information at specified intervals set by the server admin, usually 4 - 24 hours.
When changes are made to your domains information is is modified in the domain registry, which is maintained by the domain authority for domains TLD (for example Nominet maintain the registry for .UK domains) once the registry has been modified which in it's self usually takes a few hours to complete, name servers can then access the new information and the propagation process begins synchronising name server records.
Generally speaking the process takes between 12-48 hours to fully complete.
