The stuff that your
photos from the
Photomat come back on. More commonly, the stuff that you use to develope pictures onto in your
darkroom. They're composed of a base paper, and then an
emulsion of
silver halide crystals and
gelatin. The silver halide crystals turn varying shades of
gray when exposed to
light (usually from an
enlarger) and then immersed in
developer.
Ilford and
Kodak are two
popular brands. A pack of 30 8x10 sheets will run you about $20, but its worth it as long as you dont get home and find that there's a hole in the package so all the paper has already been exposed to light.