Until recently, WhatsApp only let users send text messages to their contacts alongside allowing sharing of files (including images, music or even videos). With the addition of voice calling, the app now allows voice calls over mobile data or Wi-Fi - also called VoIP or Voice over Internet Protocol.
The service is free, and not charged by WhatsApp. Much like other apps like Skype and Viber offering VoIP, WhatsApp voice calling uses a data connection to place a call, whether over Wi-Fi or mobile data. This data use however can be chargeable, based on your Internet plan.
Unfortunately, WhatsApp is rolling out the voice calling feature in phases, which means it is currently available to limited users. The company recently started pushing out the voice calling feature to all Android users with the invite window open for several hours. As of filing this report, the window has closed again, though it may open again without notice.
WhatsApp voice calling is similar in function as other VoIP apps, such as Skype, Viber, and Hangouts. However, considering that WhatsApp is the most widely used instant messaging service on mobile, having the voice calling feature in the app only expands the user experience.