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 make a call, send messages or files throw Viber. It is best used when contacts has Viber app, but Viber Out 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 screen.
Viber is sort of mix between Skype and Facebook messenger and it really useful if a lot of your contacts using it. Otherwise it has no sense. It has pictures and videos sharing function, but Video call option is lacking, whereas competitors provide this possibility.
Pros: free calls and texts using internet; easy to use; automatically synchronizes your contacts;
Cons: no video call; connection is depended on your internet connection;