WhatsApp Messenger is a cross-platform mobile messaging app which allows you to exchange messages without having to pay for SMS. WhatsApp Messenger is available for iPhone, BlackBerry, Android, Windows Phone and Nokia and yes, those phones can all message each other! Because WhatsApp Messenger uses the same internet data plan that you use for email and web browsing, there is no cost to message and stay in touch with your friends. In addition to basic messaging WhatsApp users can create groups, send each other unlimited images, video and audio media messages. -from official WhatsApp website-
WhatsApp is not very popular in the United States because most phone service packages come with free unlimited texting (SMS). However, in other parts of the world where unlimited texting may not be included, WhatsApp is very popular. Also if you have relatives in other parts of the world, this may be a great option for you. The app is free and you can download it through the Play Store (for Android devices) or the App Store (for Apple devices). Below is a link to a video giving more information on the app and what it can be used for.