Farsi and Dari are basically the same language. In Iran it's called Farsi and in Afghanistan it's called Dari. The difference between the two languages is their different accents. If you learned Spanish, you would know Castilian Spanish, which is Spoken in Spain, and South American Spanish are the same language with a few words having different meanings. But the big difference between the two languages is the pronunciations of the words. "V's and B's" are a big difference.
Farsi is the most widely spoken member of the Iranian branch of the Indo-Iranian languages.
The Farsi/Dari/Persian language has been requested.