Greek plays feature a group of performers who sing, dance, provide exposition and interact with the other characters. This group is called...

The Chorus.

Explanation:

In Greek plays, there is a group of people who would act as commentators of what the scene is unfolding, or about the characters and other details that are not really easy or possible to know from mere viewing. This group of actors help the audience by describing the scenes as they occur.

Through their performance of songs, commentaries, dancing, or even recitations, the Chorus acts as another window through which the scenes are interpreted or viewed. It provides the atmosphere for the play and also a different perspective on how the play is viewed.  

any school based questions?

lithograph is created by etching the flat surface of a print.