WhatsApp can be used to send simple text messages, but can also be used to share photos, location, plus video and voice messages. Additional features such as group chat are also available. This makes it popular, as a money saving stratagem, for those using a PAYG SIM, those on contracts with a limited message allowance, and those who frequently send messages to international numbers.
One of the highlights of WhatsApp is it easy set up; all you need to do is enter your phone number and name; you'll be asked to confirm your phone number by sending an SMS, and entering the pin code into the app that is sent back in response.
On Windows Phone WhatsApp uses a work around (related to music controls) to keep a connection open when the app is not in the foreground. While this does work, it is most definitely not what the API in question was intended for, and may have an impact on your music listening experience. We'd rather see WhatsApp make an effort to use some of the new Windows Phone 8 APIs, which obviate the need for such a hack.
Windows Store description:
WhatsApp Messenger is a smartphone messenger available for Windows Phone, Android, BlackBerry, iPhone, and Nokia phones. WhatsApp uses your 3G or WiFi (when available) to message with friends and family. Switch from SMS to WhatsApp to send and receive messages, pictures, audio notes, and video messages. First year FREE! ($0.99/year after)
Whatsapp can be downloaded for free from the Windows Phone Store.