WhatsApp Messenger is the world's most popular instant messaging app for smartphones.
You can use it to send and receive text and voice messages, photos, videos, even call your friends in other countries, and because it uses your phone's internet connection it might not cost you anything at all (depending on whether you'll pay data charges).
It's easy to set up and use. There's no need to create and remember new account names or pins because it works with your phone number, and uses your regular address book to find and connect you with friends who use WhatsApp already.
You can talk one-to-one or in group chats, and because you're always logged in there's no way to miss messages. Even if your phone is turned off, WhatsApp will save your messages and display them as soon as you're back online.
There's plenty more (location sharing, contact exchange, message broadcasting) and the app is free for a year, currently $0.99/ year afterwards.
What's New in Version 2.18.51
• When you receive a link to an Instagram or Facebook video, you can now play it right within WhatsApp. With picture-in-picture, you can also keep watching the video while you navigate to another chat.
• If you're a group admin, you can now remove admin rights from other participants. Select the admin in "Group Info" and tap "Dismiss As Admin."
• Group admins can now choose who can change a group's subject, icon and description by going to "Group Info" and tapping "Group Settings."