This is foamcore.

I've been spending a lot of time with foamcore this last couple of days, that is, the sheet foam styrene material sandwiched between cardboard oft used for model making and presenting boards on at design schools.

Anyway, it's a fun material to work with, with quite different techniques required to what I'd initially thought. I'd imaginged it was all guillotines and precise depth cutting, but it turns out its more forgiving than that, pretty good with hot glue, and much easier with a hand held knife. And with this great document from the Rolf A. Faste Foundation for Design Creativity.  The .pdf is licensed under CC 3.0 as BY-NC, and was written by the following folk:
Originally compiled by Rolf Faste and Nancy, a Stanford TA, in the late 1980s. Drawings by Peter MacDonald, 1992. Additional content by James Pierce and Haakon Faste, updated and edited 2011.

Enjoy.

PS. In case you;re wondering, the image above is of a foamcore CNC machine.