The only fix-metered, composed, non-improvisatory vocal piece is tasnif. Tasnif possesses no remarkable feature other than it is short and light. Its lyric content varies in each case, ranging from romance to socio-political content. Formerly the lyrics were composed either by the composer himself or by his contemporary fellow poet. As a general rule, in more recent examples belonging to a few past decades, the classical poetry was replaced . Tasnif is the most important and most popular of all musical genres (as compared with instrumental genres) and has also developed largely in the course of the past century. Sheyda and Aref are two of classic tasnif-composers.
Source: An Introduction to Iraninan music booklet from Mahoor Institute of Culture and Art