OK.RU is the official application of the social network Odnoklassniki, a kind of Russian Facebook used mainly in former Soviet republics such as Armenia, Moldova and Georgia (also of course in Russia).
In short, OK.RU offers a user experience and interaction very similar to that of Facebook. That is, users can publish on their walls, upload their photographs and videos, talk with other users, and so on.
OK.RU is a Russian social network, which is only in the Russian language, and therefore will only be useful if we have friends or contacts from this country or any of its satellite countries. It is also true that the social network is used, sometimes, to upload content that other websites would be difficult to find.