There are 3 main different types of Persian:
Iran Persian (Farsi)
Afghanistan Persian (Faris/Dari)
and Tajikistan Persian (Tajik)
This is because when Persia split up, it split up into those 3 countries, but mainly into Iran (although these countries were already established for a long time before Persia split up)
These Persian dialects differentiate based on country. They have distinguishable accents and a lot of words are a little bit different as well.
So in the Iranian version of Farsi uses Merci to replace the otherwise "tashakor" (that means thank you) that is used in Afghan Farsi.
I know this because I used to go to an Iranian Sunday school when I was younger in order to learn Farsi. Although I am not Iranian, and my family is Afghan, my parents decided to enroll me anyways because it was close enough. Because of this, I developed a little bit of an Iranian accent to some of my words, and say some words differently than my parents. But in all, the differences are not deadly, so two Persian speakers from different countries could continue with conversation fairly well.