Saints Cyril and Methodius, who invented famous Cyrillic and Glagolitic alphabets, worked as missionaries in South Slavic lands, and for these purposes they, naturally, need the Bible written in the aboriginal language. That is how the written Old Church Slavic appeared. It was never a colloquial tongue - no ordinary people spoke it, but all literature, church liturgy, verses and prayers were in Church Slavic. It was an official written language in Czech, Moravian, Bulgarian, Serbian, Russian and Lithuanian lands for ages, until it was replaced by Catholic Latin somewhere (in Bohemia), or by national languages (in Russia). But still, even now the official language of the Russian Orthodox church is that very Old Church Slavic, so we cannot consider it extinct or dead - it is widely used for liturgical purposes.
The language is archaic in its structure. Seven cases, including vocative,
three numbers with dual, three genders and a lot of inflections. Phonetics
has a number of differences from Russian or
any other modern Slavic language: more archaic sounds, numerous vowel and
consonant interchanges inside the stem of the word, etc. Adjectives can
be short (simple) and complex (pronominal). The most complicated is the
verb system, including many tenses such as aorist, imperfect, plusquamperfect
(past perfect), two different future tenses. Also there are conditional
and imperative moods, infinitive and supine, several kinds of participles.