WhatsApp is a cross-platform messaging app founded in 2009 by Brian Acton and Jan Koum. It allows users to send text messages, images, video, and audio media to other users without having to pay for SMS. Key features include end-to-end encryption, support for many platforms, and no login requirements. WhatsApp has grown rapidly to over 1.5 billion monthly active users as of 2016, making it one of the most popular messaging apps in the world due to its ease of use and lack of costs compared to traditional SMS.