It's a sensitive undertaking to talk about divisions in BiH. First because, obviously the History can be different according to whom relates it. Also because the media are not  always a reliable source neither, and we must be very carrefull even when we make our own researches.
 But I must mention this subject while talking about the city of Mostar, because the division is a characteristc of the city. First I should precise some general political situation in BiH: the war started in 1992, after the declaring referendum in which 99,4% of Bosnians and Croats-Bosnians approved the independance of BiH from Yugoslavia. The war finished officially in 1995 with the Dayton accord, which, according to the country's ethnic structure and changes, divided the country in two entities: the Federation of Bosnia and Herzegovina, one of the country, populated with Croats-Bosnians and Bosnians, and the Republika Sprska, north and east south part mostly populated with Serbs. Each entity has its own capital (respectively Sarajavo and Banja Luka), government, president, parliament,  and police department, TV stations...

 Aware of this, I didn't imagine that the Mostar was radically divided: each side of the river Neretva, the west part is Croatian, the East part is Bosnian  muslim.  I first began to wonder about this when a nice guy helped me to find my way and called my host. When he saw the phone number he said "it's Croatian number", and I stayed perplexed because, yes, he is Croatian, but we are now in BiH, no? I discovered then that the west part is almost Croatia: people use Croatian mobile operator, watch Croatian TV, consum e croatian product in the supermarket, young people go to Croatian university, can use there the Croatian currency Kunas, etc... Mostar has been the scene of very intense fights during the war, in this part, between Croatian and Bosnians. This went so far that in 1991, the Croatian Democratic Union, powerful political entity in Western BiH, proclaimed the Croatian Republic of Herzeg-Bosnia, that existed until 1994 before being declared illegal by the Republic of BiH. Nowadays, the Bosnian Croats represent about 15% of BiH population, and each person who declares himself as a Croatian here can particiate in Croatian political life and vote  in  the  elections of the Republic of Croatia. This system viciously leads politicians to keep on using nationalist feeling in order to get more votes, not only in Croatia...
 Marko is Croatian, and he told me since the beginning that he didn't like to talk about those subjects, but anyway it has to come in the discussion when you talk about BiH or Mostar. He studied at the Bosnian Univesity of Mostar, but never had problem. But Being a young person in Mostar doesn't seem so easy. First because of the economic situation of the country (the unemployment rate is officially 40%, that's why a huge black market has been growing and represents now about 50% of the official GDP!). And unfortunately the ethnical tensions remain , even between young people. Marko told that among his friends, Bosnians, Croats and Serbs, he knows that he can't mix some of them with each other because it will just clash.
 That's really too bad when you have the opportunity to live in a city where your can hear the pray from the mosque, the Esan, and 5 minutes later the bells of the church ringing...