Croatian language (in Croatian 'Hrvatski jezik') is one of South Slavic languages, exclusively used in Croatia. With Slovenian language (spoken in Slovenia) and Serbian language make a group of West South Slavic, while Bulgarian and Macedonian, form a group of East South Slavic.
Slovenian language is different from other two of the group and it's quite difficult to understand. Croatian and Serbian language are practically the same language and if you speak Croatian in Serbia, everyone will understand you or vice-versa you speak Serbian in Zagreb. If they wont to!
The problem is more of political nature and centuries-old conflict between these by all similar nations. The only obvious difference is in alphabet. Croatian is written in Roman alphabet and Serbs are using Cyrillic.
Croatian and Serbian used to be considered as one language (sometimes called Croatian-Serbian or Serbian-Croatian) until the disintegration of former Yugoslavia. Bosnian language is also classified within this group.
From linguistic point of view, structurally and lexical all these languages are actually so similar that can be describe as dialectical variants of the one language.
Croatian language is based on the Ijekavian pronunciation, while Serbian uses Ekavian. Difference is mostly regarding vocal 'e' in some words which is pronounced 'ije' in Croatian.
If you would like to know more about these differences, I suggest to visit UCLA - Department of Slavic, East European&Eurasian languages and cultures where they explain more detailed all language variation.
The Croatian writing system is phonetically consistent. Every letter is pronounced and its sound will not vary from word to word. Regarding to the position of stress, there is only one rule you have to follow - the last syllable of the word is never stressed. In most cases the accent falls on the first vowel in the word.
Croatian language uses a modified Roman alphabet. Many letters are pronounced like in English – but the following below may cause misunderstanding.
I must say it's not easy to learn Croatian. Grammar is rather complicated but ate least the pronunciation is not complicated as I've already mentioned. If you have no time nor inclination for Slavic languages, it will be nice to know some basic phrases how to meet, greet and thank in Croatian.
Even if you came in Croatia, without knowing a single word of Croatian, you shouldn't worry. Majority of people in Croatia, especially those along the coast, speak different languages.
If you start your visit of Croatia, from Zagreb (capital), you will be surprised that English language is spoken almost everywhere particularly by youngsters between 15 and 30.
After English language, the second one is German, spoken in Slavonija region and along Dalmatian coast. Italian language is widely used in Istria peninsula. I must proudly affirm that Croatians are multilingual.