The national language of Iran is Persian or Farsi (the two terms have the same meaning). Persian can also refer to the main ethnic group in Iran, so for clarity this website uses Iranian to refer to nationality, Persian to refer to ethnicity and Farsi to refer to language. If this seems confusing, compare it to the situation in the UK: someone may be a British national, identify as Irish and speak English. Farsi is sometimes known as Western Persian or Iranian Persian.
Farsi is written in Persian script, which is derived from Arabic script. Persian script has extra letters not found in Arabic (just as some European languages have additional letters not found in English).
Farsi is closely related to Dari, one of the national languages of Afghanistan. It is also known as Dari Persian or Afghan Persian. Dari and Farsi speakers can generally understand one another. Opinions vary on whether Dari is separate language, or a dialect of Farsi. Note that translations marked as Farsi in the UK may occasionally be in Dari, and sometimes Dari speakers will be used as interpreters for Farsi speakers. This can lead to confusion.
The Tajik language spoken in Tajikistan is also related. Some view Tajik as a separate language; others see it as a dialect of Farsi (Tajiki Persian).
Refugee Preaching Resources 