VK is the official application for the Russian Social Network of the same name, and lets you register and browse it from the comfort of your Android device. To do so, you must first enter your telephone number.
The app has a similar interface and features to Facebook. And as in any social network, to get the most out of it, you need a good group of friends that are also connected.
You can upload and share photos, videos, music, and all kinds of documents directly from the app. Likewise, you can send messages to all your friends and contacts, and see what everyone else on the network has posted.
VK is a really useful application for all the users on this network. Why would anyone become a member of a Russian social network? Well, maybe the program screenshots will tell you.