The differences are as follows:
Folk songs include folk songs and ballads. The themes are mainly ritual songs, love songs, political songs, children's songs or political satire and superstitious colors. They are all in form. Short
Long folk narrative poems generally refer to song-style stories that are circulated and preserved in oral form. It mainly includes four types: historical story songs, love story songs, magical story songs, and life story songs.
An epic is a long "composite" story song that is passed down and preserved in oral form. It is a scale that specifically describes legends or major historical events such as the origin of a nation, its migration, its wars, and the immortal achievements of its national heroes. A grand long folk narrative poem. Compared with folk narrative poems, it has, firstly, a relatively narrow subject matter and content; secondly, it has a very long length, gorgeous language, magnificent scenes and gorgeous style. There are many characters and a complex plot. Generally speaking, epics are divided into heroic epics, mythological epics and religious epics.
Such as "King Gesar"