WhatsApp is a chatting application you can say most using chatting application in world. There are so many chatting apps like viber, hike, imo but whatsApp is unbeatable. In WhatsApp you can send messages to your known person whose number is in your contact list. You can send smiles and other stickers too. You can also create group and chat in group.
WhatsApp launched calling feature few months ago but it is not that much good. I found that calling from Facebook messenger is much better than WhatsApp calling. Now WhatsApp is owned by Mark zukerberg, founder of Facebook. I suggest that if you like chatting then WhatsApp is an great application for you.