The high energy particles...

X-rays are able to penetrate different tissues in varying amounts. The difference in the X-ray's ability to pass through depends on the tissue's density.

As the X-rays encounter the human tissue they have three choices. They may pass through, bounce off, or partially penetrate the tissue. The X-ray film "counts" the number of X-rays that hit it at each position on the film. The image is built up as the number of X-rays that hit the film increases.