It's an instant message application. You also can send vídeos, pics and voice recordings. It's totally free.
To tell you the truth, it doesn't bother me. I open it when I feel like to reply.
I mentioned it before but this application is helping me to show my real me to my coworkers. You can have group people conversations.
It's funny but they know me now better vía what's app. It's easier for me to communicate with them.