Short answers, pulled from the story.
The East Slavs are a subgroup of the Slavic peoples who speak East Slavic languages and live chiefly in Eastern Europe. Their descendants extend across northern Asia to the Pacific Ocean. The existing East Slavic groups are Belarusians, Russians and Ukrainians.
The Primary Chronicle is the earliest major manuscript describing Rus' history, written in the late 11th and early 12th centuries. It lists twelve Slavic tribal unions that had settled by the 10th century between the Western Bug, the Dniepr and the Black Sea. That land became Kievan Rus'.
The split accelerated after Kievan Rus' disintegrated as a single polity in the 11th century, triggering population shifts and political regrouping. Southwestern and western Rus' came under Lithuanian and then Polish influence, while the Russian identity formed in the Muscovite northeast and the Novgorodian north.
The East Slavs relied on slash-and-burn agriculture, clearing forest tracts with fire and cultivating them before moving on after a few years. Because that method exhausts the soil quickly, it required frequent movement, which drove their rapid spread through eastern Europe.
Rusyns and Don Cossacks are sometimes considered a separate nation, though they are more often treated as a subgroup of Ukrainians and Russians respectively.
Y chromosome, mDNA and autosomal marker CCR5de132 studies show that East Slavs and West Slavs are genetically homogenous. Contrary to popular belief, they are unaffected by Turkic or Mongol influence. The one exception is Northern Russians, who cluster genetically with the Balts, Germanic and Baltic Finnic peoples instead.