Domain Name Servers (DNS) are the Internet's equivalent of a phone book. They maintain a directory of domain names and translate them to Internet Protocol (IP) addresses.
This is necessary because, although domain names are easy for people to remember, computers or machines, access websites based on IP addresses.
Information from all the domain name servers across the Internet are gathered together and housed at the Central Registry. Host companies and Internet Service Providers interact with the Central Registry on a regular schedule to get updated DNS information.
After you register a new domain name or when you update the DNS servers on your domain name, it usually takes about 12-48 hours for the domain name servers worldwide to be updated and able to access the information. These 48 hours are referred to as propagation.