A photogram is a photograph created without a camera.
Instead it's made by taking photographic paper, placing
objects on top of it, exposing it to light.
Thus the photogram is white where the light
was completely blocked, black where there was no object
in the way of the light, and various greys where
translucent objects blocked it's path.
Around 1918, photograms emerged as an art with
the creation of photograms by Christian Schad
(a member of the Dada movement
who called his photograms Schadographs), and
were used in the 1920s by photographers
such as Man Ray (who called his Rayograms).