Etymology - the study of the history of words, including their origins and how their use has evolved alongside the languages that use them. A subject I've always been fascinated in, especially concerning the English language; as a global language used in numerous contexts by millions of people, English has evolved to include thousands of words from multiple languages. While the Russian influence on English is relatively small as compared to the Romance languages, there are some interesting things linking the two.
The word 'Russia' itself is derived from the old word Rus (from Arabic Rus, Greek Rhos); allegedly it originally referred to Scandinavian/viking traders who settled in the area around Kiev, and whose descendants supposedly controlled the region; the Finnish terms for Sweden and Russia are intricately connected, derived from the root of 'rowing' (as in a boat). The Finnish apparently thought the Swedes and Russians rowed a lot.
The word 'mammoth' is derived from a Russo-Finnish term meaning earth; mammoths were believed by excavators to have been burrowing creatures who rooted under the earth. The term then came to English from Russian.
The obvious connection between the months in most indo-european languages is interesting; in fact, like the English days of the week, the Indo-European months are derived from several sources, including numbers (october/октябрь, derived from 'octo' or 'eight'; the old Roman calendar had 10 months, October being the eighth), and a shared Indo-European mythology (or mythical references borrowed from Latin; I'm not sure where the Russian month names come from, but its possibly from Latin.) March and January come from Roman gods (Mars and Janus, respectably).
While historic language and the evolution of the Proto-Indo-European culture into the numerous cultures and languages it is today is incredibly fascinating, one could (and many have) fill entire books with interesting connections and observations.