WhatsApp Messenger is a proprietary, cross-platform instant messaging application for smartphones. In addition to basic messaging WhatsApp Messenger users can send each other images, video and audio media messages.
WhatsApp Messenger is available for iPhone, BlackBerry, Android, Symbian and Windows Phone only (and on the iPad unofficially). WhatsApp synchronizes with the phone's contact's book, so users do not need to add contacts in a separate book.
WhatsApp is a temporarily free application in iOS. In other platforms, the app is free for BlackBerry, but Android and Nokia users must pay yearly after the first year.
The name WhatsApp is a pun of the popular phrase "What's up?"
Source: Wikipedia, "WhatsApp Messenger", available under the CC-BY-SA License.