It's not that uncommon to link from your site to others. For example, if I am writing an article on diabetes, I would probably want to link to various supporting articles from the Mayo clinic or other valid resources on the web. Doing this is actually quite simple.

The first thing you need to do is highlight the text you want to link from (if it's an image you want to link from, highlight the image by single-clicking on it). Now, click on the "Link" button on the Inspector, and then check the box titled "Enable as hyperlink", as shown below:

After you have checked the box, you will notice that the options below it become active. If you click on the drop-down box to the right of "Link To:", you will see four options:

One of my Pages
When you choose this option, a new drop-down box appears allowing you to choose an existing page from your current site. If you want to link to a page that has not been created yet, you would need to begin at least a template for that page, save it, then link to it (making the changes later).

An External Page
This option allows you to link to an external site such as www.cnet.com or www.apple.com/displays.

A File
This option is particularly cool when sharing documents (such as a spreadsheet or word processor document). When you choose this option, you will be prompted to browse for the file to link to. Note that when this is done, a copy of the file is uploaded with the site; it is not opening the file directly from your Mac.

An E-mail Message
If you're showing photos of your new baby or a house project, it would be nice to have your e-mail address linked in there somewhere so that people can send you an e-mail directly from the page. Using this option allows you to do this easily. When choosing this option, you will be prompted to enter an e-mail address and subject.

Simply choose the option that suits your needs the best, make sure that the appropriate parameters are set (i.e.: web address, e-mail address, etc), and hit Enter when you are done.