In Afghanistan, Dari is the official language used in government. Pashto is also commonly used, and predominantly spoken by Afghans in the southern part of the country.
Dari stems from Farsi, spoken in Iran and others "stan" countries. There is about a 25% difference between Dari and Farsi. This is a fairly big difference between the two languages.
Dari Learning Resources
If you are heading towards Afghanistan as a military contractor, soldier, consultant, aid worker, or diplomatic or Embassy personnel, you may still find the Rosetta Stone software for Farsi a helpful purchase.
Some of the best information can be found at Interlit Foundations. This publishing company are the experts at producing language learning materials for Afghanistan - both for Afghans and for expatriates.
Dari Greetings and Politeness
There are a few phrases helpful to learn in order to build friendships with people who speak Dari, or Farsi for that matter. The most important first phrases to learn are the greetings.
In Persian cultures, politeness is extremely important. Politeness is communicated through the initial greetings when meeting someone. It is common just to "run through" the greetings super fast, not really waiting for much of an answer, although you should also answer. You should ask these questions at least twice, but three times is better.
In English, it is like this:
Hello. How are you? How is your health? How is your family? Are you well? Is life well? Is your family well?
Answer: Thank you, I am well. My family is well. God is good. Very well, thank you.
In Dari, the translation:
Salaam, Chetor Asten? Jan Joras? Familetan khubas? Shuma Khub estan? Zenda geetan khubas? Familetan khubas?
Then answer: Tashakur, Khub astum. Familema Khubestan. Nam-e-Khuda. Besyar Khub. Tashakur.
Take time giving the greetings with your Persian friend - they will be delighted with your attempt, and will appreciate your efforts. Don't be embarrassed - one way to describe even bumbling efforts in Middle Eastern culture is to picture "Giving joy to the culture" even when you make a mistake!