4/0: A printed piece with full color printing on the front.
4/1: A printed piece with full color printing on the front and black printing on the back.
4/4: A printed piece with full color printing on both sides.
Insert: The printed piece that goes in the front cover of a jewelcase.
Traycard: The printed piece that goes behind the disc in a jewelcase.
Panels: The total number of squares that make up a printed piece. A basic CD insert has 2 panels (one square panel on the outside and one on the inside). A basic folded insert has 4 panels (2 on the outside and 2 on the inside).