what do you mean by between slavic and russian? russian is a slavic language so I think you wanted to ask the difference between russian and all other slavic languages. 

russian is an east slavic language so it will be most similar to the other east slavic languages. If you learn russian youll most likely be able to understand some of Ukrainian and Belorussian. It will only help you minimally to learn the other slavic languages, but you will have a better grasp of them if you know russian, the grammar will be familiar to you ect. However, if you want to learn russian, learn russian to know russian not another slavic language.