Newsboard is a form of compressed cardboard often made from recycled newsprint (thus the name). it is commonly used in matting pictures, mounting or backing artwork, etc.
I use it for the covers of my books as it's affordable and easy to work with since it comes in 1/4th inch thickness.
However - my main supplier has "chosen" not to carry it any longer so I'm on the hunt for a new source. Unfortunately I have several projects that require a lot of it (double folds take 3 boards alone).
More when I find a better source.