When you type in www.definitehosting.co.za for example, how does your computer know how to find www.definitehosting.co.za? It asks the DNS servers.
The DNS servers (also known as name servers) tell browsers where to find a site. (It is actually far more complicated than that, but that is 80% of their function).
Also when you register a new domain the domain name has to propogate through the internet and all DNS servers, thats why it takes a couple of hours before your domain name is active and available on the internet.