The iPhone offers three special kinds of links that receive special treatment. Each of these links can appear in Safari or in your mail. When clicked, they tell the iPhone to launch a specific application to handle them.
The first, the mailto: link, you're probably already familiar with. It's just like the Web-based mailto: link that's been in use for the last decade. When clicked, it opens your iPhone's email application, creates a new message and addresses it to the target of the link, e.g. mailto:email@example.com.
The second link is tel:. As you might guess, it opens the iPhone's calling application and calls the number used as the link's target. This allows Web developers to add "call us" links in their web page that, when tapped, actually place a call.
The third kind involves Google Maps. Instead of opening Google Maps links in Safari, they automatically open in the iPhone Maps widget instead. It doesn't matter whether you click them in Safari or Mail. The iPhone recognizes the link and launches the Maps widget for viewing.