graphic printing is done on an 11x17 or larger sheet, from which several tray cards and inserts are cut out.
once cut, a 2 panel insert is a piece of paper folded once and inserted into the front of the jewel case. once folded, it is therefore 2 panels, 4 3/4" square, which is 4 pages if you count the front and back sides, just like a book. We can do up to 16 pages, that's 8 panels.
a tray card and spine label are the paper which goes inside the jewel case between the back side of the jewel case outer box and the tray that holds in the cd. The spine label is the 1/4" wide edge of the jewel case you see from the side when it is on a shelf with other cds, and is created when the edges of the tray card are creased and folded up at 90 degrees upon insertion into the jewel case.
if the inside cd tray is clear instead of black, you can see through it to the back side of the tray card. With a clear tray, even when the jewel case is closed, you can see a small strip throught the front of the jewel case for about 1/2" at the left side. This is good for putting a vertical title, or special graphic that you see inset in the cd case.
a 4/1 insert and tray card means back side of the printed sheets are grayscale (1 colour, k (black) on back side). That is the most common CD order since the inside of an insert isn't seen unless you take out the insert and unfold it. Grayscale is still photographic quality, like a black and white photograph. Once inserted therefore, you see colour on the outside, and on the inside when you open the jewel case, and see the black and white only if you take out the insert and unfold it, or underneath the cd tray if you want a clear tray and grayscale printed on the tray card back side.
you can get colour on both sides of the printed sheets if you want thats called a 4/4 insert. (4 colour, cmyk, meaning cyan, magenta, yellow, black: the four colours used to make up the full colour spectrum in offset printing and laser printing processes) the tray card would also therefore be colour both sides if wanted.