The Slavic languages are a branch of the Indo-European languages languages; they and the Baltic languages together form their own macro-branch (that is, the two of them diverged from Proto-Indo-European before they diverged from each other).
The earliest Slavic literary language is Old Church Slavonic, which was/is a Southeast Slavic language probably closest to modern-day Bulgarian (although nationalism makes this a contentious claim). It’s still used as the liturgical language of some Orthodox churches. Its heyday was the 9th to 11th centuries, and the many written records in OCS provide important evidence to linguists today about the features of Proto-Slavic and, thus, the evolution of the Slavic languages as a whole.
The two main alphabets in which Slavic languages are currently written are Cyrillic in the east, and the Latin alphabet in the west. The Cyrillic alphabet was developed specifically for Old Church Slavonic, so some consider the alphabet basically “purpose-built” for the Slavic family (optimised for their phonologies). Historically other alphabets were used too, like the Arabic alphabet, which was sometimes used to write Serbo-Croatian during the time of the Ottoman Empire.
Slavic is quite a conservative branch of Indo-European, retaining much of the highly inflectional, fusional morphology of PIE. (The exact amount differs between languages, of course, with those languages highly influenced by the Balkan sprachbund having lost a little more.)
The Slavic languages can be subdivided as follows (noting that there are smaller, minority languages not included in the list below – this is not exhaustive):
- East Slavic
- West Slavic
- South Slavic
Note that the division between East and West Slavic is based mainly on extralinguistic factors (primarily the alphabet each group is written in). Ukrainian, for example, has a higher degree of lexical similarity with Polish (70%) and Slovak (66%) than Russian (62%). The grammars of all the East and West Slavic languages are very, very similar. The South Slavic group is more distinct, especially the southeastern subgroup.
The Slavic languages are thought to have started breaking up comparatively later than other branches of the Indo-European family, with Proto-Slavic existing up until 500 CE. As a result, the Slavic languages remain closer to one another than the languages of many other Indo-European branches. There is a considerable degree of mutual intelligibility between neighbouring languages, but between more distant language pairs things gets harder. Still, often basic communication is possible if people speak slowly, and try multiple synonyms if a word is not readily understood. There is a constructed language, Interslavic, which aims to be a simplified and readily understandable interlanguage to facilitate communication between native speakers of different Slavic languages.