Scholars recognize three major dialect divisions of Persian: Farsi, or the Persian of Iran, Dari Persian of Afghanistan, and Tajik, a variant spoken in Central Asia. All are more or less mutually intelligible.

Dari, mainly spoken in Afghanistan, until recently, deferred to the Tehran standard as its model, and Farsi and Dari remain quite similar. The dialectal variation between Farsi and Dari has been described as analogous to that between European French and Canadian French. Tajik is slightly less similar; Farsi and Tajik have been compared to American and Australian English in terms of mutual intelligibility.

Dari and Farsi are written with a modified Arabic script. Tajik is written with a Cyrillic script. Here are samples:

DARI

FARSI

TAJIK
Тамоми одамон озод ба дунё меоянд ва аз лиҳози манзилату ҳуқуқ бо ҳам баробаранд. Ҳама соҳиби ақлу виҷдонанд, бояд нисбат ба якдигар бародарвор муносабат намоянд.

(Article 1 of the Universal Declaration of Human Rights, as cited by http://omniglot.com. Map and description by UCLA Language Materials Project (http://www.lmp.ucla.edu/Profile.aspx?LangID=55&menu=004).)