The following is a detailed explanation.
Generally speaking, the titles of Yuefu poems are songs, lines, quotations, songs, songs and so on. This kind of songs and songs are all Yuefu themes, and there are also poems that lead to resentment. They are based on Yuefu proposition. Except for Qin music, they are all called long-winded and unreasonable songs, like modern prose essays. They are more important than the galloping of notes, more important than lyric and generous songs, and the classification is not particularly obvious. For example, Li Bai's three-five-seven-word zaqu is a style test of ancient poetry. Compared with modern poetry, even rhyme is not particularly strict.
Yuefu, as its name implies, was originally an institution in charge of music. The Yuefu organization was formally established in the period of Qin and Han Wudi. Its task is to collect and edit local folk music, arrange, adapt and create music, and perform singing and performances. Later, people called the poems collected and created by this institution Yuefu Poetry, or simply Yuefu Poetry. By the Tang Dynasty, although the scores of these poems had long been lost, this form followed, becoming a poetic genre with no strict meter and close to five or seven words. In the Tang Dynasty, poets wrote Yuefu poems. Some people used old Yuefu poems to write current events to express their feelings, such as "Song of the Sage" and "Guan Shanyue". Some people wrote new poems to reflect real life.
In Song Dynasty, Guo Maoqian collected all Yuefu poems and compiled Yuefu poems, such as Mulan Poetry (Southern and Northern Dynasties).
The Yuefu poems in the two Han Dynasties were originally sung by music. The reason why they are different from the four-character quatrains in the Book of Songs in poetic form is not only the inevitable result of the development of poetry itself, but also the musical factor.
The so-called new Yuefu is relative to the ancient Yuefu. This concept was first put forward by Bai Juyi. He has compiled more than 50 poems, such as Beauty is Better than Happiness and Setting a Topic for Something, into New Yuefu. The new Yuefu has three characteristics: one is to use new questions. Since Jian 'an, most of the songs written by literati are based on current events and follow ancient topics, often with limited content and inconsistent topics. Bai Juyi writes current affairs with new topics, so it is also called "New Topic Yuefu". The second is to write current events. After Jian 'an, writers have their own innovative themes, but most of them have nothing to do with current affairs. There are both new themes and current events, Gu Du Fu. Bai Juyi followed the tradition and stabbed the reality with the new Yuefu. Third, it is not measured by whether you are happy or not. Many new Yuefu poems are to taste the "songs broadcast in the movement". Musically, it has the name of Yuefu, but in content, it directly inherits the realistic spirit of Han Yuefu and is a real Yuefu. The forerunner of the New Yuefu Movement was Du Fu, followed by Yuan Jie and Gu Kuang, and finally Zhang Ji and Wang Jianwei. During the period of "Bai Yuan", it clearly put forward a whole set of theories of "combining articles with the times, and combining songs and poems with phases". In addition, Yuan and Bai poems were the best in the world, and a large number of new Yuefu poems appeared, which had a great influence at that time, making this great literary movement a great success.