Like other Middle Eastern music, the music of Iran is modal in nature. Initially (before the Qajar dynasty) each of the major modes had an associated formula for melodic invention (mayeh). The mayeh included rules for cadences, a heirarchy 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 twelve dastgahs. Each dastagah has an associated eight note scale, and each tone in the scale has a special significance, with one note being designated the analogue of the tonic in Western diatonic music. The dastgah also has its own repertory of melodies, each of which is called a gushe. A gushe is actually a melodic type which usually spans only four or five tones, and serves as a model for improvisation. Generally the gushe are played in an order which fills the lower, middle, and upper portions of the dastgah scale. Aside from that, the order and mode of each gushe may not have a logical relationship to that of the dastgah itself. The different gusheh are bond together by melodic fragments known as foruds, which inevitably resolve to the finalis of the dastgah. Within each dastgah are also encoded the rules for achieving that resolution. The initial gusheh in a dastgah is called the daramad, and it lends its name to the dastgah. Thus the dastgah-e-Shur is that dastgah which has the modal melody Shur as its daramad.
These points may be illustrated by examining the layout of the dastgah-e-Shur. The modal structure (the eight tone scale) of Sur is presented below:

Sur

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Note that the scale does not span an octave per se, as it is bound by a b semi-flat on its lower end and by a b-flat on the upper end . Also, the 5th above finalis is played as an A during ascending melodic movement, while it is lowerred by a microtone in descending melodies. The bracketted whole notes show the tetrachord within which the main melodic activity takes place. Melodic movement is strictly diatonic, and leaps larger than a perfect 4th are not made within a phrase. The 2nd below finalis is the aqaz, or the point from which improvisation is initiated.
Two formulae for the daramad of Sur are presented below. Again, these formulae serve only as the basis for improvisation, and many dastgah-e Sur pieces have two daramads, one based on each formula.

Two Daramads in Sur

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The daramad of Shur from Asheq'i peidast in Classical Music of Iran, Smithsonian Folkways 40039. CD #869 (498K WAV)
Click the speaker to hear a daramad to the dastgah-e-Shur, as it is renderred in a perfromance. This example demonstrates the way in which a typical performer treats the formulae that are part of the dastgah.
There are several prescribed routes to a forud in Sur. The finalis may be approched from (a) the 2nd below, (b) the 3rd then 2nd below, (c) the 2nd above, or (d) the 4th above. Again these rules serve as the basis of improvisation and the foruds may therefore vary in length and type from performer to performer.

The four routes to a forud in Sur

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The gushehs of dastgah-e Sur are: Salmak, Molla Nazi, Golriz, Bozorg, Xara, Qajar, Ozzal, Sahnaz, Qarace, Hoseyni, Bayat-e Kord, and Gereyli. The order of gushes within a dastgah is not fixed, and some gushes may be omitted altogether.
The melodic formula of Salmak is presented below:

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Gusheh Salmak from Asheq'i peidast in Classical Music of Iran, Smithsonian Folkways 40039. CD #869 (417K WAV file).

As is demonstrated in the above example, these formulae serve only as a basis for improvisation, although the musicians are expected to render each gusheh in such a way that it remain identifiable.

Finally, the combiantion of all pieces that make up the repertory of Persian music is called the radif (row). Thus, the radif of Persian music contains the twelve dastgahs: Shur, Abu Ata, Dashti, Bayat-e Tork, Afshari, Segah,Chahargah, Homayun, Bayat-e Esfahan, Nava, Mahur, andRrast, with all of their constituent gushehs.

Information provided by http://www.duke.edu/~azomorod/dastgah.html