What is the difference between "Што" and "Шта"?
What is the difference between "Што" and "Шта"? When I say "What is this?", which form is used? "Što je to?" или "Šta je to?"

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ШТО и ШТА depend of the context of the sentence.  ШТО is a short version of ЗАШТО. ШТА can be a question for the subject, but also can be a question, but not a subject. 
Example: Шта радиш? Шта гледаш? Шта читаш? ...
You're example Што је то? is Croatian and Шта је то? is in Serbian. 
Sentences are the same, but in different languages.

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што could be WHY ( zašto)   and  šta is hat.

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Hi Ruslan! In Russian or in Serbian?

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Hello, "Shta" isjust incorrect and slang unofficial form of "Chto" that was created in memes.
Young people use it in case when one's surprised and he/she asks "Shta???"