WHATSAPP is an application for smartphones
WHATSAPP is a messaging service, a chat program for mobile phones actually. This allows you to start conversations with other users, you can also send pictures or movies back and forth. The program has become popular because it is used as an alternative to SMS. Because WhatsApp is available on any mobile operating system, it doesn't matter which phone your friends have, with WhatsApp you can always send messages without the expensive cost of SMS. WHATSAPP uses the Internet connection (i.e. from Wi-Fi or from your data bundle), so the costs are many times lower (or free) than SMS.