Viber is communication tool that uses your phone number to contact other people for free.
Viber application uses 3g internet or WiFi for connection. You can call, send messages or files throw Viber. It is best used when contacts has Viber app, but Viber could be used for applicants without an app, it sometimes might be cheaper then calling directly.
Viber has nice function as you can pin your most important conversation to your start.
Pros: free calls and texts using internet; easy to use;
Cons: no video call; connection is depended on your internet connection;