Yugoslavia was a haphazardly made state consisting of many slavic ethnicities. Tito was its leader and was generally able to keep the country together despite massive tensions between ethncities, between the USSR and the West, it was one of the few communist countries that was not under direct USSR rule.
When Tito died, Serbians managed to get the most influence in the government and army and thought this was the best time to carve out a Greater Serbia and went all genocide-y on others, realizing that Yugoslavia was probably not gonna survive long without someone to ease tensions between the different peoples. It was so utterly bad and such unspeakable shit was done that NATO got involved and bombed the shit out of Serbia.
When Niko Bellic from GTA 4 talks about cutting the hands off kids and literally selling people and children, he is talking about the shit he did in that war.