Your topic is a bit big. You can write a book if you explain it in detail. To put it simply, there is no essential difference between ancient poems, words and songs, but their forms and styles are different. Ancient poetry tends to be stable, suitable for expressing ambitions, with simplicity as the top priority, and the so-called ingenuity does not work, and the epee has no front; Words tend to be light, suitable for telling feelings, exquisite and smooth, and turn thousands of times; However, the songs tend to be transparent, pay attention to rhyme, be easy to understand and use straightforward words.
In a word, from ancient poetry to ci, from ci to qu is from regularity to flexibility, from refined scholars to the public, which is a process of poetry popularization and an inseparable whole.