Ci (词) originated in the Tang and the Five Dynasties, and developed to maturity as a new literary form in the Song Dynasty. Also known as “lyric with a melody,” “yuefu (乐府) poetry” or “long and short verses,” ci developed from poetry. Its main feature is that it is set to music and sung. Each piece of ci has a name for its tune. There are strict requirements for the number of lines and the number of characters as well as tone pattern and rhyming in different tunes. In terms of length, ci is divided into short lyrics, medium lyrics, and long lyrics. In terms of musical system, a piece of ci is usually divided into two stanzas of que (阕) or pian (片), as ancient Chinese called them. Occasionally, it consists of three or four stanzas, or just one. Thus, the music can be played once or many times. In terms of style, ci falls into the graceful and restrained school and the bold and unconstrained school. The former is delicate and sentimental, often describing family life and love, while the latter is bold and free, often expressing one’s vision about major social issues like the fate of the nation. Many literati and scholars of the Song Dynasty composed ci lyrics, which played a significant part in promoting its development. Today, ci is generally not set to music and sung. Rather, it is a literary form composed in accordance with the requirements of a music tune.
Ci means lyrics written for music, whereas qu is musical tunes set to accompany lyrics. (Liu Xizai: Overview of Literary Theories)
During the Song and Yuan dynasties, ci and qu were one and the same thing. When written with words, they were ci; when composed with music, they were qu. (Song Xiangfeng: Epilogue to Yuefu Poetry)