Russian is the most geographically widespread language of Eurasia, the most widely spoken of the Slavic languages, and the largest native language in Europe. Russian belongs to the family of Indo-European languages and is one of three living members of the East Slavic languages, the others being Belarusian and Ukrainian (and possibly Rusyn, in Ukraine considered a dialect of Ukrainian and recognized in some countries).
The Russian language is primarily spoken in Russia, Ukraine, Kazakhstan and Belarus, and, to a lesser extent, the other countries that were once constituent republics of the USSR. During the Soviet period, the policy toward the languages of the various other ethnic groups fluctuated in practice. Though each of the constituent republics had its own official language, the unifying role and superior status was reserved for Russian. Following the break-up of the USSR in 1991, several of the newly independent states have encouraged their native languages, which has partly reversed the privileged status of Russian, though its role as the language of post-Soviet national discourse throughout the region has continued.
Russian is the official language of Russia. It is also an official language of Belarus, Kazakhstan, Kyrgyzstan, and the unofficial but widely spoken language in Ukraine. There are several opinions on how many dialects that Russia has. Some say two and some say three all based on regions.
If you get ambitious you could try Russian, once you have learned Russian you can begin to communicate with other Slav speakers.