Serbo-Croatian is a pluricentric South Slavic language often known by the name of one of its standardised forms (Croatian, Bosnian, Serbian or Montenegrin). In total, it has about 21 million speakers. It was the primary language of Yugoslavia before its disintegration.
Historically, the three primary dialects of Serbo-Croatian were Shtokavian, Kajkavian and Chakavian, named after each dialect’s word for “what” (što, kaj and ča respectively). Shtokavian became the most widely-spoken and prestigious dialect over time, and so all the modern standardised languages are derived from it. Chakavian today is spoken by about 80,000 people in coastal Croatia, while Kajkavian is spoken by some in north-central Croatia (including in and around the capital, Zagreb); it is said to be more like standard Slovenian than standard Serbo-Croatian. To varying degrees (i.e. Shtokavian more than the others), the dialects form part of the Balkan sprachbund.
One of the main isoglosses subdividing the Shtokavian dialect involves the treatment of the Proto-Slavic vowel “jat” (ě). Some Serbo-Croatian dialects pronounce this /i/, others /e/, and others still as /ije/ or /je/. Winning my heart with their straight-to-the-point names, these dialect groups are described as “Ikavian”, “Ekavian” or “Ijekavian”. Standard Croatian and Bosnian are based on Ijekavian pronunciations, while in Standard Serbian officially both Ijekavian and Ekavian are acceptable.
Serbo-Croatian is quite a highly inflected language, with seven grammatical cases and a pretty complex verb conjugation system. Word order is flexible but SVO by default. It is a pro-drop language, meaning subject pronouns are included only for emphasis.
Phonetically, vowels in Serbo-Croatian can be either short or long and pronounced with either a rising or a falling tone, giving rise to a pitch accent; while this results in four total combinations, most speakers do not differentiate between all four. Most speakers in Serbia and Croatia don’t distinguish between short rising and short falling vowels, for example, and many speakers in Zagreb don’t distinguish tones at all. Whether a vowel is stressed or unstressed also makes a difference (many speakers treat most unstressed vowels as short, except to avoid ambiguity).
Serbo-Croatian can be written either in the Latin alphabet or the Cyrillic alphabet, with a 1:1 correspondence between graphemes in each alphabet. The Croatian and Bosnian standardised forms are always written in Latin, while Standard Serbian can be written equally in either (although the government itself prefers Cyrillic for official documents). Serbo-Croatian’s orthography is quite straightforward and phonetic.