Greek theaters or otherwise know as outside theaters housed the plays of Greek times.  Early Greek Theaters would have performing plays in open areas or next to hillsides where the viewers standing or sitting viewed and listened to the singing of the exploits of a God or Hero.  From the 6^th century on theaters gradually evolved to more elaborate building however the layout of the theater remained the same.   

Orchestra:

The area found in the middle of the theater would normally have a round shape. Performers would sing and play along with other the performers on stage near the skene in this area.  Hard earth formed the earliest round area of Greek theater.  Marble formed the same area as time evolved and other materials t as well, you may have also found an alter in this area. The Middle area of Dionysus in Athens measured 60 feet.

Theatron:

Otherwise know as the viewing area is where the viewers sat. The area overlooked the middle part of the stage and often wrapped around a large portion of the middle part of the theater.  In the earlier theater the viewers sat on padded seats or boards, in later time many theatrons had marble seats.

Skene:

Directly behind the building you would find the skene disguised as a temple or other building depending on the needs of the play.  It had at least one set of doors and performers would go in and exit through them.  The roof would also be available through the skene so that performers playing God and other parts would sometimes appear on the roof.

Pardos:

The Parodoi or otherwise know as passageways got used by some performers and viewers to enter and exit the theater before and after the play.