Arabic and Farsi are two different languages entirely; the fact that they use the same alphabets does not make to be the same. Also, the fact that there are some words and idioms that are common to both languages doesn't make them be the same either. Farsi, the standard Parisian language, belongs to countries like Iran, Tajikistan, and Afghanistan. Arabic, on the other hand, is usually regarded as a Semitic language, just like Hebrew.
It is a language originating from the Arabian Peninsula, and most countries, especially in the Middle East and some parts of Africa, are now using it as their native language. Also, while the two languages have common words, they don't have the same pronunciation for those words; their grammar and vocabulary are not even the same. Also, while every alphabet in Arabic can be found in Farsi, the latter has four extra alphabets that cannot be found in Arabic.