Once you’ve downloaded and installed it, you have to introduce your phone number and the app will automatically find the contacts in your phonebook who are already using Whatsapp. These will be the contacts you can talk to on Whatsapp.The strength of the application lies, firstly, in its “cross-platform” nature: this means that it can be used from Android, iPhones, Symbian and RIM (Blackberry) devices. So you can have conversations with all your contacts, no matter what OS they're using! This kind of integration is what users really appreciate. Secondly, its interface is simple, easy and attractive (including the Facebook profile pic of your contacts, if you have Facebook integrated with your phonebook). This is another important reason why most people are using Whatsapp instead of other mobile messenger services. And thirdly, it has plenty of great features: you can create a list of block contacts, create chat groups on a specific topic, chat settings (font size, type mode, your nickname, status message), and notifications settings (ringtone for specific contacts, vibration, pop-up,...).One of its flaws is the price (and it is not really a flaw since it's cheap, but it could cost Whatsapp a few users). Android users get a Whatsapp free trial for a year, then they have to pay $2 for it. It works on Android 2.0 and later but we've noticed some lag problems in older devices.Whatsapp was developed by Whatsapp Inc., a promising app development company located in Silicon Valley. Whatsapp is its first app, and its success is well-known among smartphones users.If most of your contacts are using it, Whatsapp is a must-have for your Android. Forget SMS/MMS forever; the free cross-platform messenger era has arrived.