The Viking Age Baltic societies were the groups of Iron Age tribes living along the shores of the Baltic Sea. They would have been one of the factors which shaped the practices of the Vikings and their contemporary societies.
Polabian Slavs from Poland and eastern Germany used longships, similarly to the Norse, and defended their territories with fortresses called Gords. Referred to as Wends (or Vindr in Old Norse) by Northmen, they were divided into tribes, such as the Obotrites, the Veleti, the Rani, the Havelli, and the Lutici. Tribes were led by chieftains of princely status called Knes (related to the Russian title of Knyaz). After Knes came the Voivot, the chief warrior and governor of a district ruled by the tribe. Helping the Knes govern the people was the council of landowners called the Veche.
The Kurs or Curonians were an ethnic Balt tribe occupying coastlands of modern Lithuania and Latvia. Much as their Norse Viking counterparts, they were noted as fierce warriors and skilled sailors.
The Oeselians were a Balt tribe who inhabited the Estonian island of Saaremaa. The Oeselians were formidable pirates. The were referred to in Snorri Sturlusson's Heimskringla as Vikingr fra Esthland (Vikings from Estonia). In the late 10th century C.E., the future Norwegian king, Olaf Tryggvason, was taken captive by Oeselians Vikings during boyhood.
Kvens were a Finnish tribe found in was is now Finnland and the northern Swedish region of Norrland. Some groups herded reindeer, while others were hunter-gatherers. Many lived as nomads, though some located on the Finnish shores of the Gulf of Bothnia may have adopted agriculture through contact with the Norse. They may have sailed in smaller boats and raided the shores of Svealand and Götaland.
While under Earl Haraldson's rule, the Earldom of Kattegat (and most of the other Viking earldoms and kingdoms) used to raid the Baltic Sea and fight against the Slavs, who had a tribal lifestyle and lived in small villages, always fighting each other. They were also pagans, although they had their own religion(s) and worshiped different gods than the Norsemen. They were also much poorer than the western Christian peoples, like the Saxons and Franks, and even poorer than the Norsemen, as stated by Rollo in the pilot episode Rites of Passage. The Balts make their only appearance in the series in the Norsemen-Balts Battle at the start of the pilot episode.