Baltic scientists in their research[2] worked out a model of Slavic lexical identity. In the picture, you can see the percentage of common lexicon provided by connections between different Slavic languages.
There are three Slavic linguistic and ethnic branches - East Slavic (Russian, Belorussian, Ukrainian and Rusyn), West Slavic (Czech, Slovak, Polish, Kashubian, Silesian, Upper-Sorbian, Lower-Sorbian) and South Slavic (Slovenian, Serbian, Croatian, Macedonian and Bulgarian).
This division goes back into history. Different communication events with other nations, geographical position, territorial pecularity — everything affected Slavic people that became nations that now exist. It is supposed to have three initial tribes of Slavs: Venedes (the ancestors of West Slavs), Antes (the ancestors of East Slavs) and Sklavins (the ancestors of South Slavs). It is controversial, but a viable and a rather popular model within Slavic community.