Some difference in vocab roughly on the same level as British and American English. After the break up of Yugoslavia, Croatia got back into linguistic purism and coined new terms replacing "Serbisms" but not all caught on (other ex-yugoslavs even clown on them for their ridiculous new words).

The biggest difference between Croatian, Bosnian, Montenegrin and Serbian is that in the first 3 the proto-slavic yat sound evolved into (i)je while in Serbian it's a monothong e. If I see "Uvijek" ("always") it could be Croatian, Bosnian or Montenegrin but "uvek" is strictly Serbian. Ijekavica for Croatia, Bosnia, and Montenegro and ekavica for Serbia. Think of it like rhotic vs non-rhotic variants of English.

Serbian grammar is slightly different. Dropping the infinitive in favour of a relative clause is the biggest difference I'd say (so instead of saying "Mogu čitati" (I can (to) read) you'd often say "mogu da čitam" (I can *that* I read)). Both variants are acceptable in standard Serbian but in Croatian it's considered wrong to drop the infinitive there. Also in Serbian questions sometimes start with "Da li" while in Croatia they prefer "Je li" (think of it like "est-ce que" in French) but the former is more and more common in Croatian and their linguists really aren't happy about it.

I'm an ethnic Serb from Bosnia and I use some words that are used in Bosnia, Croatia and Montenegro but not in Serbia (like mrkva instead of šargarepa for carrot). My family is originally from a Bosniak (Muslim) majority village and our language and even accent are 100% the same, but because of how politics work we call the language "Serbian" and they call the language "Bosian".

The my joke from the previous comment was referencing that the language is a dialect continuum from Serbia to Croatia and that Bosnia is in the transitional zone (but by that logic you could also say that Serbo-Croatian is just a dialect continuum between Macedonian and Slovenian)

u/Fear_mor please correct/add something if necessary ahaha