Well there are several ways:
NOTE: This information is given under the assumption that all requests are http - other things (like non-webmail) are handled VERY differently!
1. If the address in the URL bar stays the same even when you click to other pages, you are using
frames. This is the
worst approach as it does not allow screen readers and people with text-only/REALLY old browsers to view your page.
How it works: In essence, your NS sends all content from yourdomain.com silently to a page on their website that contains a <frameset> tag pointing to your domain.
2. If your host is using these things called A Records, your URL will change when you click on links in your site. What it does is whisper to the browser "I know you have typed in yourdomain.com but you should really get content from freewebs.com/yoursitewithlonguglyurl"!
