All good answers, other than to add a couple points:
Ancient Rus gave birth to an Eastern Slavic civilization that became an immense beacon for nearly all Slavs (and some non-Slavs) too far east to experience the tug-of-war between Constantinople and Rome but when Rus imploded and was eventually carved up by the Mongols/Tartars and Lithuania, many Eastern Slavic groups were cut off from one another and regional differences became increasingly pronounced. By the 19th century there were still many, many more Eastern Slavic groups than there are today. The Soviets in particular 'consolidated' and eliminated or ignored many groups long recognized by Lithuanians, Poles, Hungarians, Turks and Austrians; Lenin simply organized them into two main groups: Byelorussians (White Russians) and Ukrainians. Gone were Red Ruthenians, Black Ruthenians, the Lemko, etc. Rather than distinct groups, these peoples - some of which still survive today - are seen now as mere linguistic dialects of Russian, Ukrainian or Belarussian and not languages or cultures in their own right. 19th century nationalism destroyed or marginalized a lot of groups like this all across Europe.
Re: the Germans of Russia; as Russia gained control over the Baltics in the 17th and 18th centuries it inherited a large number of Germans (especially in the territories of modern Latvia and Estonia), who were descendants of medieval German colonists who had spanned throughout the eastern Baltic lands. Some derived from the post-crusades military orders (the Livonian Order, the Teutonic Knights) in the 12th and 13th centuries, but most had derived from farmers and merchants who had migrated to the Baltics for opportunistic reasons from c. 12th to the 16th centuries. Riga (the modern Latvian capital), for instance, was an early and prominent member of the Hanseatic League. Some of 18th and 19th century Russia's great generals were Baltic German barons. Indeed, as I recall the principal Russian general at Tannenberg in 1914, Rennenkampf - perhaps not the most auspicious example - was a Baltic German.