difference between the two languages. What might that difference be?

I've no direct personal knowledge on Slavic languages, but my dad worked in what was then Yugoslavia for a few years, and he lent me a phrase book published by Collins that confirms the general outline like this:

There is no actual “Yugoslav” language; in fact the major tongue of the country is Serbo-Croat, Serbian being written in Cyrillic and Croat in Latin characters. There are slight differences between these two languages but the traveller need have no fear of being misunderstood when [speaking].

As a matter of interest, Serbian is spoken in the eastern and Croat in the western parts of Yugoslavia. Two other minority languages exist; one is Slovenian which is similar to Croat and also written in Latin characters and is encountered in the north-west regions. The other is Macedonian which is half-way between Serbian and Bulgarian and, like Serbian, is written in the Cyrillic characters and used in the far south.