They are Belarus, but long time ago they traveled south to live in lowlands of Panonic sea (that's Danube river, most of Hungary, parts of Croatia and Serbia).
Here they live in villages as majority, but minority within country...after all centuries passed they are no longer Belarus, have their own culture and language (altough similiar to original one)...think Shakespearean to modern english...
They call themselves Rusini, as opposed to either Russian or Belarus.
That's all I know...I lived in the city and had fairly limited contact with father's kin. I can understand them, but cannot speak the language. And since my father dies (almost 20 years ago) I had essentialy no contact with them...