Chat with your friends using text, stickers and emojis, send photos back and forward, make high-quality audio and voice calls and transfer the calls between your devices.
The Internet connects people. But in order to be connected, people need tools. There are various messaging apps like Skype or ICQ around, but they lack one thing - familiarity. They are a bit alien to people, and that is why Viber has the advantage here.
What is Viber? It's a program that makes your Internet an extension of your telephone. Basically, the Viber client syncs your web connection to your telephone database and lets you make calls, send text messages, transfer files with functionality similar to modern phones, but with the advantage of a computer. Video calls are available, making chatting and communicating a lot more interesting, especially with the option to join public chats and find new people to talk to. But the main feature of Viber is the ability to transfer the ongoing call you have on your phone to the PC in order to cut the costs or turn on additional features.
Viber clearly originated as a mobile app, but grew to be something more. It's easy to use, easy to look at and useful to people who like to talk and have a lot of friends. Unfortunately, it doesn't offer much else in terms of functionality, but its main purpose is fulfilled in spades.