Links demonstrate the interconnectedness of concepts and institutions. Links are also used to provide a means to navigate within a single page that is more effective than using the scrollbar.
Links can be external or internal.
The two pages could be part of a single website, or it could be a link between a page in your website and a page in somebody else's website. Most websites are made up of more than one page. In each case, the principle is identical. In general, you should avoid using underlining as a tool for formatting words or sentences because of the way that WWW browsers use underlining to assist readers to identify a page's links. Netscape is a great program, but it does tend to freeze up and crash the computer at the worst moments.