Whats app is best chat application ever, fast, very user friendly and very very usefull , most of the user here I think wont require much of an introduction to whats app but still its my tribute to a great app.
First we used to use sms feature and use to buy so many sms packs which at that time was great.
After the introduction of the whats app there is no competion in this niche , it just have dominated.
Its not only a chat app its beyond that now due to the introduction of instamoji you can now also receive payments from whats app and aquisition from facebook meaning that its a giant now.