KOI8-RU is an 8-bit character encoding, designed to cover Russian, Ukrainian, and Belarusian which use a Cyrillic alphabet.
It is closely related to KOI8-R, which covers Russian and Bulgarian, but replaces ten box drawing characters with five Ukrainian and Belarusian letters Ґ, Є, І, Ї, and Ў in both upper case and lower case.
It is even more closely related to KOI8-U, which does not include Ў but otherwise makes the same letter replacements.
Although this may seem unnatural, it has the useful property that if the eighth bit is stripped, the text can still be read (or at least deciphered) in case-reversed transliteration on an ordinary ASCII terminal.
For instance, "Код Обмена Информацией" in KOI8-RU becomes kOD oBMENA iNFORMACIEJ (the Russian meaning of the "KOI" acronym) if the 8th bit is stripped.