Wikipedia
Yuefu are Chinese poems composed in a folk song style. The term originally literally meant " Music Bureau", a reference to the imperial Chinese governmental organization(s) originally charged with collecting or writing the lyrics, later the term yuefu was applied to later literary imitations or adaptations of the Music Bureau's poems. The use of fu in yuefu is different from the other Chinese term fu that refers to a type of poetry or literature: although homonyms in English, the other fu is a rhapsodic poetry/prose form of literature.
The term yuefu covers original folk songs, court imitations and versions by known poets (such as those of Li Bai). As opposed to what appears to be more of an authentic anonymous folk verse which was collected by the Music Bureau, verse written deliberately in this style, often by known authors, is often referred to as "literary yuefu". The lines of the yuefu can be of uneven length, reflecting its origins as a type of fixed-rhythm verse derived from now lost folk ballad tunes; although, later, the five-character fixed-line length became common. However, as a term of classification yuefu has a certain elusiveness when it comes to strict definition. Furthermore, the literary application of the term yuefu in the modern sense of a classical form of poetry seems not to have had contemporary application until considerably after the end of the Han Dynasty, thus adding a certain historically ambiguity due to its use in this literary sense not having occurred until centuries after the actual development of this type of verse itself. The use of the term yuefu to generically refer to this form of poetry does not seem to appear until the late fifth century CE.