A large traditional arched staging which is located at the near or front of the stage is called a Proscenium stage. The use of the term "proscenium arch" is explained by the fact that in Latin, the stage is known as the "proscenium", meaning "in front of the scenery." Ordinarily the audience tends to be raised several feet above front row audience level, and view the performance through the proscenium "arch" which is often has decorative architecture. The main stage is the space behind the proscenium arch, often marked by a curtain which can be lowered or drawn closed. The stage is used mainly for drama, plays, musicals, concerts etc.
This is a video made of images of proscenium theatres and stages I found off Youtube.