Russian is a Slavic language in the Indo-European family. From the point of view of the spoken language, its closest relatives are Ukrainian and Belarusian, the other two national languages in the East Slavic group that are also descendants of Old East Slavic. Some academics also consider Rusyn an East Slavic language; others consider Rusyn just a dialect of Ukrainian. In many places in Ukraine and Belarus, these languages are spoken interchangeably, and in certain areas traditional bilingualism resulted in language mixture, e.g. Surzhyk in eastern Ukraine. An East Slavic Old Novgorod dialect, although vanished during the fifteenth or sixteenth century, is sometimes considered to have played a significant role in formation of the modern Russian language.

The vocabulary (mainly abstract and literary words), principles of word formation, and, to some extent, inflections and literary style of Russian have been also influenced by Church Slavonic, a developed and partly adopted form of the South Slavic Old Church Slavonic language used by the Russian Orthodox Church. However, the East Slavic forms have tended to be used exclusively in the various dialects that are experiencing a rapid decline. In some cases, both the East Slavic and the Church Slavonic forms are in use, with slightly different meanings. For details, see Russian phonology and History of the Russian language.

Russian phonology and syntax (especially in northern dialects) have also been influenced to some extent by the numerous Finnic languages of the Finno-Ugric subfamily: Merya, Moksha, Muromian, the language of the Meshchera, Veps etc. These languages, some of them now extinct, used to be spoken right in the center and in the north of what is now the European part of Russia. They came in contact with Eastern Slavic as far back as the early Middle Ages and eventually served as substratum for the modern Russian language. The Russian dialects spoken north, north-east and north-west of Moscow have a considerable number of words of Finno-Ugric origin.[2][3] The vocabulary and literary style of Russian have also been greatly influenced by Greek,[citation needed] Latin,[citation needed] Italian,[citation needed] French,[citation needed] German,[citation needed] Ukrainian,[citation needed] and English.[citation needed] Modern Russian also has a considerable number of words adopted from Tatar and some other Turkic languages.

According to the Defense Language Institute in Monterey, California, Russian is classified as a level III language in terms of learning difficulty for native English speakers,[4] requiring approximately 780 hours of immersion instruction to achieve intermediate fluency. It is also regarded by the United States Intelligence Community as a "hard target" language, due to both its difficulty to master for English speakers as well as due to its critical role in American world policy.