KOI8-R is the character set of choice for encoding Russian texts for many Unix-like operation systems. KOI8-R is a successor for KOI-8, a de-facto standard for Internet Mail, News, WWW and other interactive services at least all over the ex-SU territory.
KOI8-R was designed for mixed Russian/English texts and covers only Russian Cyrillic characters, so if you're looking for Ukrainian, Byelorussian, etc. Cyrillic characters, try ISO-IR-111, or KOI8-U (Ukrainian Character Set), or KOI8-C (for ancient Russian texts) instead, which are identical to KOI8-R in the Russian Cyrillic letters area.
A more complete set of Cyrillic characters is also defined by the ISO-8859-5 character set.