Iran is a very big country with variety of ethnicities and races. From region to region you hear many different accents and there are at least 4 main languages and 3 main dialects of Farsi.
Tehrani accent is a Dialect of modern Persian Language spoken in Tehran Province, and the most common colloquial variant of the modern Persian Language.
Compared to literary standard Persian, the Tehrani Dialect lacks original Persian diphthongs and tends to fuse certain sounds.
Tehrani accent of Persian Language should not be confused with the old Tehrani Dialect, which was a northwestern Iranian Dialect, belonging to the central group.
Iranians can interchange colloquial Tehrani and standard Iranian Persian sociolects in conversational speech.
References:
http://en.wikipedia.org
http://www.quora.com
