The common consensus is that Belarusian, Russian and Ukrainian are the extant East Slavic languages.
At the same time, Belarusian and Southern Russian form a continuous area, making it virtually impossible to draw a line between the two languages.
It was also the native language of the Cossack Hetmanate until the end of the 18th century, when the Ukrainian state completely became part of the Russian Empire in 1764.
The alphabets of the East Slavic languages are all written in the Cyrillic script, however each of them has their own letters and pronunciations.
Additionally, Belarusian and Ukrainian use the apostrophe (') for the hard sign, which has the same function as the letter Ъ in Russian.
Other examples: B. ваўчыца (vaŭčyca) U. вовчиця (vovčyc’a) ”female wolf” B. яшчэ /jaˈʂt͡ʂe/ U. ще /ʃt͡ʃe/ “yet” /u̯/ (at the end of a closed syllable) B. стэп /stɛp/, U. степ /stɛp/ "steppe" B. Вікторыя (Viktoryja) U. кобзар (kobzár (nominative case) кобзаря (kobzar’á (genetive case) R. кровь (krov’), кровавый (krovávyj) B. кроў (kroŭ), крывавы (kryvávy) U. кров (krov), кривавий (kryvávyj) ”blood, bloody” B. скажа (skáža) U. скаже (skáže) ”(he/she) will say” After the conversion of the East Slavic region to Christianity the people used service books borrowed from Bulgaria, which were written in Old Church Slavonic (a South Slavic language).
It has been suggested to describe this situation as diglossia, although there do exist mixed texts where it is sometimes very hard to determine why a given author used a popular or a Church Slavonic form in a given context.
Church Slavonic was a major factor in the evolution of modern Russian, where there still exists a "high stratum" of words that were imported from this language.