Slavs
Slavs are the largest ethno-linguistic group in Europe. Present-day Slavic people are classified into East Slavs (chiefly Belarusians, Russians, Rusyns, and Ukrainians), West Slavs (chiefly Czechs, Kashubs, Moravians, Poles, Silesians, Slovaks and Sorbs), and South Slavs (chiefly Bosniaks, Bulgarians, Croats, Macedonians, Gorani, Pomaks, Montenegrins, Serbs and Slovenes).
Church Slavonic language
Church Slavonic, also known as Church Slavic, New Church Slavonic, or New Church Slavic, is the conservative Slavic liturgical language used by the Orthodox Church in Bulgaria, Poland, the Czech Republic and Slovakia, Russia, Belarus, Serbia, Montenegro, Bosnia and Herzegovina, the Republic of Macedonia, Ukraine, Slovenia, and Croatia. The language appears also in the services of the American Carpatho-Russian Orthodox Diocese and occasionally in the services of the Orthodox Church in America. It was also used by the Orthodox Churches in Romanian lands until the late 17th and early 18th centuries, as well as by Roman Catholic Croats in the Early Middle Ages.
It is also co-used by Greek Catholic Churches, which are under Roman communion, in Slavic countries, for example the Croatian, Slovak and Ruthenian Greek Catholics, as well as by the Roman Catholic Church (Croatian and Czech recensions, see below).