Like other Middle Eastern music, the music of Iran is modal in nature. Initially (before the Qajar era) each of the major modes had an associated formula for melodic invention (mayeh). The mayeh included rules for cadences, a hierarchy of tones, and acceptable melodic patterns. Using the mayeh as a guideline, the musician was expected to improvise within a single mode for the duration of the performance, much as is done with Indian raga. Gradually, this method became cumbersome for the musicians and for the listeners.
As a result, during the Qajar dynasty, the old modes and mayehs were restructured and the dastgah system was developed. The modes were replaced by the seven dastgahs and five avazs. The names of these dastgahs are: shur, mahur, rast panjgah, segah, chargah, nava, homayun. The avazs are: abuata, bayat-e turk, afshari, dashti and esfahan. In studying Persian music theory this point should be taken into consideration that in Persian music like some other modal musical systems, tetrachords and penatchords are more important than scales.
Persian music is a modal music in which different combinations of tetrachords and pentachords make its modes. As mentioned, all of these modes are categorized into a large repertoire known as the radif, classified in large units known as dastgahs and avaz. In fact, the radif is the model repertoire with modal, rhythmic, and melodic potentialities anticipated for Persian classical music. As a matter of fact, the repertoire, while being extremely flexible, is used as a model for improvisation and composition.
The radif itself has smaller parts which are dastgah, avaz, and gushe. In fact, the dastgah also has its own repertory of modes and melodies, each of which is called an avaz and gushe. Avaz is a sub-dastgah whose structure is very similar to that of a dastgah but in smaller scope.
For more information's about theory of persian music see the link below:
http://www.farabisoft.com/Pages/FarabiSchool/TheoryDetails.aspx?lang=en&PID=6&SID=36