www.viber.com/dl is a download link for the Viber application. Most people find it in an invitation SMS (text message) sent by their friends.
Hey, you should get Viber. It's a free iPhone app that I use to make free phone calls – http://www.viber.com/dl
If you click on it with your iPhone (OK, tap on it), it will start the “App Store” application with the Viber page already opened. This represents a kind of a platform where you can see info, reviews, screen shots and from here you can install Viber as well.
When it comes to Android devices, the result is basically the same, only with one obvious difference – it points to the “Android Market”, of course.
If you choose to open that link in your browser, this will lead you to the Viber’s homepage.
Interestingly, if you open that link with a Blackberry or any other mobile device, it will open a page saying that “Viber is NOT compatible with your device yet”.