Cool page with lot's of info, sound samples, graphs etc.
" 1. The background of the Common Slavic language.
Actually, no signs, inscriptions, historical documents or even short glosses remained from the Common Slavic language, as well as from Proto-Indo-European, Common Celtic or many other proto-languages. Slavs did not have writing when they lived together and spoke their Common Slavic language. Archaeological research can provide us with names of different archaeological cultures found in East Europe and in European Russia, they describe the way of life of those ancient tribes, their pottery and iron working, but tell nothing about the language. And so comparative linguistics is the only subject which can prove the existence of such a language as Common Slavic, and even rediscover its phonetic and morphological structure.
Slavs must have come from one source - that's the first thesis which even does not require proofs. As Slavic peoples nowadays speak relative languages it is evident they used to have one common one. Was it like Russian (East Slavic), Bulgarian (South Slavic), Polish (West Slavic)? It was not any of them, and at the same time was similar to each of them. Old Polish, preserved in medieval manuscripts (see Slavic Links), shows much more similarity to Old Russian spoken in Kiev Russia, than the modern Polish does to modern Russian. The Old Church Slavic, the ancestor of nowadays Bulgarian and Macedonian, looked quite like Old Russian and Old Polish together. But even they fifteen centuries ago were different and could be misunderstood by each other relative Slavic nation. The earlier level of development of all them was Common Slavic."
Common Slavic Language