Folk music
It is music of, for and by people (folk).
For me, it is a shared experience.
So I'm with Eliza on this one.
At true folk events, singing along is encouraged.
And speaking as a performer, the most magical moments often happen when you hear a song 'sung back at you' - often with the most sublime harmonies.

More singing at concerts please. If you want to hear me in perfect clarity (more fool you), rather than be immersed in the folk experience, then buy the CD.