To add to the previous educators answer....
After Josep Broz Tito died in 1980, the new leader was Slobodan Milosevic. An ethnic Serb, like Tito, he ruled Yugoslavia right before the USSR collapsed officially in 1991. Many former satellite countries, free of the USSR, started to declare independence. The six regions of Yugoslavia (Bosnia&Herzegovina, Croatia, Slovenia, Serbia, Montenegro, and Macedonia) started to break away.
The biggest problems came when Bosnia&Herzegovina declared independence. Unlike in Slovenia and Croatia, where most of the population were the same ethnicity, Bosnia was a hotbed of different ethnicities. The largest three ethnicities in Bosnia in 1993 (at the time of independence) were the Bosnians (Muslims), Croats (Catholics), and Serbs (Christians). Yugoslavia, a Serbian-run state, moved in to 'protect' the Serbian minority in Bosnia. What occurred here was later coined "ethnic cleansing", or one ethnic/religious group getting rid of another ethnic/religious group using terror tactics (ranging from sexual assault, torture, and murder to imprisonment and forced deportation) to remove the latter ethnic/religious group from a country/region. In this case, the Serbs attacked the Croats and Muslims in Bosnia, trying to rid Bosnia of the other ethnic groups. Why? To hopefully have a region that was fully Serbian and would likely stay part of the now smaller Yugoslavia.
For a time, there was little international response. This changed after bombings at Sarajevo, the Bosnian capital, raised the alarm in the Western world. NATO (North Atlantic Treaty Organization) stepped in, bombing Bosnia. The Croats and Muslims were also launching a counter-attack on Milosevic's Serbian paramilitary and government forces, causing Milosevic to sign a peace treaty.
The end result: 200,000 Muslims and Croats ethnically cleansed as a result of Yugoslavia breaking apart.