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, Montenegrins, Serbs and Slovenes) .
Slavs can be further grouped by religion. Orthodox Christianity is practiced by the majority of Slavs. The Orthodox Slavs include the Belarusians, Bulgarians, Macedonians, Montenegrins, Russians, Serbs, and Ukrainians and are defined by Orthodox customs and Cyrillic script as well as their cultural connection to the Byzantine Empire (Serbs also use Serbian Latin script on equal terms). Their second most common religion is Roman Catholicism. The Catholic Slavs include Croats, Czechs, Kashubs, Moravians, Poles, Silesians, Slovaks, Slovenes, and Sorbs and are defined by their Latinate influence and heritage and connection to Western Europe. There are also substantial Protestant and Lutheran minorities especially amongst the West Slavs, such as the historical Bohemian (Czech) Hussites.
The third largest religion amongst the Slavs is Islam. Muslim Slavs include the Bosniaks, Pomaks, Gorani, Torbeši, and other Muslims of the former Yugoslavia as well as certain East Slavs who settled in the Crimean Peninsula and converted to the Islamic faith via influence from the Crimean Tatars. Modern Slavic nations and ethnic groups are considerably diverse both genetically and culturally, and relations between them – even within the individual groups – range from ethnic solidarity to mutual hostility.
The oldest mention of the Slavic ethnonym is the 6th century AD Procopius, writing in Byzantine Greek, using various forms such as Sklaboi (Σκλάβοι), Sklabēnoi (Σκλαβηνοί), Sklauenoi (Σκλαυηνοί), Sthlabenoi (Σθλαβηνοί), or Sklabinoi (Σκλαβῖνοι), while his contemporary Jordanes refers to the Sclaveni in Latin. The oldest documents written in Old Church Slavonic, dating from the 9th century, attest the autonym as Slověne (Словѣне). These forms point back to a Slavic autonym which can be reconstructed in Proto-Slavic as *Slověninъ, plural Slověne.
The reconstructed autonym *Slověninъ is usually considered a derivation from slovo ("word"), originally denoting "people who speak (the same language)," i. e. people who understand each other, in contrast to the Slavic word denoting German people, namely *němьcь, meaning "silent, mute people" (from Slavic *němъ "mute, mumbling"). The word slovo ("word") and the related slava ("glory, fame") and slukh ("hearing") originate from the Proto-Indo-European root *ḱlew- ("be spoken of, glory"), cognate with Ancient Greek κλέος (kléos "fame"), as in the name Pericles, Latin clueo ("be called"), and English loud.
Ancient Roman sources refer to the Early Slavic peoples as Veneti, who dwelled in a region of central-Europe east of the Germanic tribe of Suebi, and west of the Iranian Sarmatians in the 1st and 2nd centuries AD. The name Veneti derives from Latin venus, -eris ('love, lovable, friendly'). The Slavs under name of the Antes and the Sclaveni make their first appearance in Byzantine records in the early 6th century. Byzantine historiographers under emperor Justinian I (527–565), such as Procopius of Caesarea, Jordanes and Theophylact Simocatta describe tribes of these names emerging from the area of the Carpathian Mountains, the lower Danube and the Black Sea, invading the Danubian provinces of the Eastern Empire.
Jordanes in his work Getica (written in 550 or 551 AD). describes the Veneti as a "populous nation" whose dwellings begin at the sources of the Vistula and occupy "a great expanse of land." He describes the Veneti as the ancestors of Antes and Slaveni, two early Slavic tribes, who appeared on the Byzantine frontier in the early 6th century. Procopius wrote in 545 that "the Sclaveni and the Antae actually had a single name in the remote past; for they were both called Sporoi in olden times." The name Sporoi derives from Greek σπείρω ("I scatter grain"). He described them as barbarians, who lived under democracy, and that they believe in one god, "the maker of lightning" (Perun), to whom they made sacrifice. They lived in scattered housing, and constantly changed settlement. Regarding warfare, they were mainly foot soldiers with small shields and battleaxes, lightly clothed, some entering battle naked with only their genitals covered. Their language is "barbarous" (that is, not Greek-speaking), and the two tribes do not differ in appearance, being tall and robust, "while their bodies and hair are neither very fair or blond, nor indeed do they incline entirely to the dark type, but they are all slightly ruddy in color. And they live a hard life, giving no heed to bodily comforts..." Jordanes described the Sclaveni having swamps and forests for their cities. Another 6th-century source refers to them living among nearly impenetrable forests, rivers, lakes, and marshes.