WhatsApp is a third party messaging app used worldwide in lieu of traditional SMS text messaging. It is a free service, although a '$1.99 per year' subscription was once considered.
WhatsApp first arrived on Windows Phone back in September, 2011. Since then, the app has had regular updates to keep up with the changes to the Windows Phone platform.
During its early development stage on Windows Phone, the WhatsApp company worked closely with Nokia and its developer partners. The goal was to make (and keep) WhatsApp a competitive messaging client on the struggling Windows Phone platform.
WhatsApp is used extensively in emerging markets where traditional text messaging is cost prohibitive. Penetration levels in the US is comparatively small to other countries, like India or Brazil. WhatsApp's biggest competitors are Facebook Messenger, Skype, and LINE.
Benefits in using WhatsApp over SMS messaging consists of zero cost, including sending messages internationally, the ability to send unlimited photos and videos, group chat and presence awareness/read-message indicators.
Voice calling (VOIP) is expected to arrive on WhatsApp for Windows Phone sometime in 2015. The voice feature was announced in early 2014 for all platforms, but due to unexpected delays it has been pushed back until Sprint 2015.
A private beta of WhatsApp for Windows Phone sometimes reveals new and upcoming features before they are released to the main, public branch of the app.