WhatsApp is a cross-platform messaging app founded in 2009 and based in California. It is available for iPhone, BlackBerry, Windows, Android, and Nokia. Key features include no login/logout required, no international charges, support for multimedia like photos and videos, and group chat functionality. It has over 400 million active users and processed over 54 billion messages on New Year's Eve. While it has strong brand loyalty and market leadership, WhatsApp faces threats from competition from other messaging apps.