www.viber.com/dl is the download link for the Viber application. Most people will find it in their invitation SMS (text message) sent by 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 tap on the link with your iPhone, it will start the “App Store” application with the Viber page already opened. Here you can see info, reviews, and screenshots of the app, and from here you can install Viber as well.
When it comes to Android devices, the process is basically the same, only with one obvious difference – it points to the “Android Market”, of course.
If you choose to open the link in your browser, this will lead you to 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”.