The Rus (Old East Slavic: Рѹсь, Old Norse: Garðar) are an ethnic group in that formed the Kievan Rus. They were originally Norse people, mainly originating from Sweden. These Norsemen merged and assimilated with Slavic, Baltic, and Finnic tribes.
In the mid-800's A.D. they were centered around Novgorod. They dominated the area for over a hundred years. The Rus were led by Oleg, who captured Kiev, founding the Kievan Rus. The descendants were the ruling dynasty of Rus.
The Rus never attempted to conquer Scandinavia and held reasonably friendly relations with their Norse cousins. Things deteriorated first in the Middle Ages when Scandinavia became Catholic and Russia became Orthodox, and more importantly as their expansions started to collide. Some historians consider the portrayal of the Rus in the series as the first time the show's artistic license with history becomes downright offensive.
Their armor, helmets, and the fact that they seem to be immensely skilled horsemen is basically the Turko-Mongol empire of the Golden Horde centuries too early. Also, Prince Oleg's primary enforcer is a giant man with Mongol-style loop braids and a huge furry hate called Ganbaatar. While Turkic/Khazar bodyguards would be perfectly acceptable for this time period, Ganbaatar is a Mongol name (meaning something similar to "Steel Hero"), and the Mongols are way too far to the east to be known to the Russians at this time.
Early-stage Russia in the series is already rife with the various modern stereotypes of Russia. Also, they're around a century too early with their Christianization, which was only fully completed in the 10th century, almost simultaneously with that of their supposed West Norse cousins.
In real life, the Kievan Rus were but one relatively minor Slavic-Norse power in the western edges of the Eurasian Steppe. They were bordered by far more militarily and economically powerful Turkic states such as the Volga Bulgars, Khazar Khanlig, or Pechenag Oghuz. The political context in which they lived actually drove them to adopt many aspects of Turkic political culture as a means to establish themselves as credible peers to these various states, hence why the early Kievan Rus were not ruled by Princes or King, but by Khagans. In addition to this, Turkic mercenaries were prized among the early Rus rulers for their combat skills and so it would not have have been out of the ordinary at all for Rus rulers to have a continent of steppe nomads to counter the forces of the Khazars or Bulgars.