DNS propagation is the process by which the computers on the Internet update their records to reflect new Domain Names and site names. Once your Domain Name has propagated throughout the Internet, your Domain Name can be accessed. The process can take from 24-72 hours.
The Domain Name System is a global network of servers that translate host names into IP addresses. Most DNS servers are UNIX machines running BIND (Berkely Internet Name Domain) software; other platforms and software are available. The software is composed of two elements: the name server and a resolver. The name server responds to browser requests by supplying name-to-address conversions. This information is cached (saved for a predefined period of time). When/if it doesn't know the answer, the resolver will ask another name server for the information. It continues to look up the name servers until it finds a name server that knows the answer. Occasionally browsers can timeout waiting for this process even when the propagation has taken place.