Russian (ру́сский язы́к, russkiy yazyk, pronounced [ˈruskʲɪj jɪˈzɨk] ( listen)) is an East Slavic language and an official language in Russia, Belarus, Kazakhstan, Kyrgyzstan and many minor or unrecognised territories. It is an unofficial but widely-spoken language in Ukraine, Latvia, Estonia, and to a lesser extent, the other countries that were once constituent republics of the Soviet Union and former participants of the Eastern Bloc. Russian belongs to the family of Indo-European languages and is one of the three living members of the East Slavic languages. Written examples of Old East Slavonic are attested from the 10th century onwards. 

Russian distinguishes between consonant phonemes with palatal secondary articulation and those without, the so-called soft and hard sounds. This distinction is found between pairs of almost all consonants and is one of the most distinguishing features of the language. Another important aspect is the reduction of unstressed vowels. Stress, which is unpredictable, is not normally indicated orthographically though an optional acute accent (знак ударения, znak udareniya) may be used to mark stress, such as to distinguish between homographic words, for example замо́к (zamok, meaning a lock) and за́мок (zamok, meaning a castle), or to indicate the proper pronunciation of uncommon words or names.
What ones?
Ukraine