It's all about DNS (Domain Name System). When you register a domain name, the registrar creates a record that includes the IP addresses of the primary and secondary DNS servers for your domain. Those domain servers are where you define the name and IP address of the server that hosts your site, as well as the IP address of the mail server that handles your mail.
Large companies have their own DNS servers, so when they register, the record is setup to point to their internal DNS servers. If you register through a web hosting service like GoDaddy, they'll specify their shared DNS servers in your registration and pre-configure a record to point to your website at
www.yourdomain.com. They also give you the ability to edit the registration record so you can point it to different DNS servers, like DynDNS.com, and they give you an on-line tool to add new DNS records or change the existing ones.
Candlepower Forums, for instance, is currently registered through Network Solutions which assigned the following DNS servers:
NS67.WORLDNIC.COM 205.178.190.34
NS68.WORLDNIC.COM 205.178.144.34
A record on those DNS servers defines
www.candlepowerforums.com to have IP address 72.167.36.24 which is a server hosted by GoDaddy.