WhatsApp Messenger is a proprietary

Proprietary software

Proprietary software is computer software licensed under exclusive legal right of the copyright holder. The licensee is given the right to use the software under certain conditions, while restricted from other uses, such as modification, further distribution, or reverse engineering.Complementary...

, cross-platform

Cross-platform

In computing, cross-platform, or multi-platform, is an attribute conferred to computer software or computing methods and concepts that are implemented and inter-operate on multiple computer platforms...

 instant messaging

Instant messaging

Instant Messaging is a form of real-time direct text-based chatting communication in push mode between two or more people using personal computers or other devices, along with shared clients. The user's text is conveyed over a network, such as the Internet...

 application for smartphone

Smartphone

A smartphone is a high-end mobile phone built on a mobile computing platform, with more advanced computing ability and connectivity than a contemporary feature phone. The first smartphones were devices that mainly combined the functions of a personal digital assistant and a mobile phone or camera...

s. In addition to basic messaging WhatsApp Messenger users can send each other images, video and audio media messages.

WhatsApp Messenger is available for iPhone

IPhone

The iPhone is a line of Internet and multimedia-enabled smartphones marketed by Apple Inc. The first iPhone was unveiled by Steve Jobs, then CEO of Apple, on January 9, 2007, and released on June 29, 2007...

, BlackBerry

BlackBerry

BlackBerry is a line of mobile email and smartphone devices developed and designed by Canadian company Research In Motion since 1999.BlackBerry devices are smartphones, designed to function as personal digital assistants, portable media players, internet browsers, gaming devices, and much more...

, Android, Symbian

Symbian

Symbian is a mobile operating system and computing platform designed for smartphones and currently maintained by Accenture. The Symbian platform is the successor to Symbian OS and Nokia Series 60; unlike Symbian OS, which needed an additional user interface system, Symbian includes a user...

 and Windows Phone

Windows Phone

Windows Phone is a mobile operating system developed by Microsoft, and is the successor to its Windows Mobile platform, although incompatible with it. Unlike its predecessor, it is primarily aimed at the consumer market rather than the enterprise market...

 only (and on the iPad

IPad

The iPad is a line of tablet computers designed, developed and marketed by Apple Inc., primarily as a platform for audio-visual media including books, periodicals, movies, music, games, and web content. The iPad was introduced on January 27, 2010 by Apple's then-CEO Steve Jobs. Its size and...

 unofficially). WhatsApp synchronizes with the phone's contact's book, so users do not need to add contacts in a separate book.

Users must buy WhatsApp if they wish to use it in iOS - 69p in the UK store. 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?"

Security concerns

In May 2011 there was a security hole reported in WhatsApp which left user accounts open for hijacking.

Communications made by WhatsApp are not themselves encrypted, data is sent and received in plaintext, meaning messages can be easily read if packets can be sniffed.

In September 2011 a new version of the WhatsApp Messenger application for iPhones was released. In this new version, the developer has closed a number of critical security holes that allowed forged messages to be sent and messages from any WhatsApp user to be read.