Dom, it's simp[ly the address of the site to be linked to, wrapped up in a way that can be easily "clicked" on. So, if you wanted to direct someone to a site, simply typing the site address would result in that address becoming the link. The difference with a hyperlink is that it uses code to warp it up, or present it in another, or simpler, way.
The address of Google's site is
http://www.google.co.uk and that will appear as an underlined link in any programme that recognises it as such. If I wanted to use a single word as the link, I need to enclose the address in code, thus:
Code:
[url=http://www.google.co.uk][b]Google[/b][/url]
which will appear as
Google (blue still, but not underlined until you hover the mouse over it) but actually works as a link.
Hyperlinks are not only used on the Internet - they appear in many Help sections of other programmes, linking one topic to another, or to suggest related topics, for example. Other programmes, Microsoft Word for one, have an automated method for creating hyperlinks. If you have MS Word on your machine, click on Help and search for "hyperlink" to see how it's done.
HTH
Ray.