Republika Srpska and the Federation of Bosnia and Herzegovina are the two entities of Bosnia and Herzegovina. That's strange and a result of the Bosnian War. However, this area is more or less the northern part of the country and also at the southeastern border. So actually it has two regions which are not connected. Together with Bosnia and Herzegovina the country has almost triangular shape.
Republika Srpska, both parts, is part of the Dinarides and heavily karstified. There is a watershed, and despite being so close to the Mediterranen Sea most of the region actually drains to the Black Sea. The region has numerous exceptional karst feature and great caves. The Trebišnjica is one of the longest sinking rivers in the world. It belongs to the Adriatic Sea drainage basin.