WhatsApp Messenger is a proprietary cross-platform instant messaging client for smartphones that operates under a subscription business model. It uses the Internet to send text messages, images, video, user location and audio media messages to other users using standard cellular mobile numbers.

As of September 2015, WhatsApp had a user base of up to 900 million, making it the most globally popular messaging application.

WhatsApp Inc., based in Mountain View, California, was acquired by Facebook Inc. on February 19, 2014, for approximately US$16 billion.

Source: Information extracted from Wikipedia in Oct 2015
Picture by Álvaro Ibáñez (Creative Commons by 2.0)