Photograms are photographic images made (without a camera) by placing objects directly onto the surface of a photo-sensitive material such as photographic paper and then exposing it to light. The result is a silhouetted image varying in darkness based on the transparency of the objects used, with areas of the paper that have not received any light appearing light and those that have appearing dark, according to the laws of photosensitivity. The image obtained is hence a negative and the effect is often quite similar to an x-ray.
In the digital photography world the equivalent to this process would be called scan-o-grams. The process works essentially the same as photograms but the objects are placed on a flat bed scanner. This process is sometimes also called "scanner as camera".
Here are some examples of both photograms and scanograms. Can you tell the difference?