The differences between Persian from Iran as opposed to Afghanistan/Tajikistan are mostly due to vowel pronunciation, and certain word usage.

For example, if one wants to describe a quality or large amount (e.g. the word "really" like in the phrase really cool, or "very" in thank you very much) an Iranian would use "<em>khaily</em>" whereas Afghans would most likely use "<em>bisyar</em>".

in terms of pronunciation, like I stated earlier it has a lot to do with the how the vowels are pronounced in specific words. A good example would be the words for milk and lion.

Dari Persian retained a lot of the originial pronunciations for words so milk (sheer) and lion (shehr) are pronounced differently. However, in Iranian Persian, the vowels in both words are the same so they are pronounced milk (sheer) and lion (sheer), and are differentiated depending on the context of the conversation.

Once you understand that the dialectical differences are vowel-related, once you start hearing Dari Persian (if you're learning Iran Persian) or visa versa, you will notice the differences.

Hope that answers your question a little bit.

Dari is a kind of Farsi(Persian) that is spoken in Afaghnistan and East-Iran . Persian has three kinds :

1- Falati or Farsi

2- Dari

3- Tajiki

all of them are very similar and speakers of one kind can understand others, differences are in some accents and non-important words. Dari has more ancient words of farsi whereas Falati has a lot of arabic words. Tajiki is combined with Middle Asian Languages Like Hindi & Mongoli. But there's no Huge difference.

If you want to understand all of them, you can simply learn Dari and then  with a little attention you can understand the rest

after all i'm natively a Falati Speaker :)

زیادتر در کشور ایران زبان قارسی و در کشور افغانستان و تاجکستان زبان دری صحبت میشود........

Sohrab jan,

Falati ye esme dige baraye zabane Parsi, o ye zabane digast? Chon man hich vaght nashanidam kasi began "falati" bare Parsi.