Both visible light and X-rays consist of photon particles conveyed over electromagnetic energy. When carefully controlled, this energy can be used to reveal the internal structure of the body. We find these pictures quite helpful:
Interestingly, X-rays were discovered soon after the first chiropractic adjustment in 1895. Wilhelm Roentgen, a German physics professor, was experimenting with cathode ray tubes and discovered that an object across the room began to glow. Much to his surprise, when he placed his hand between the tube and the glowing barium screen, he was able to see the bones of his hand.
From those crude beginnings over a century ago, both chiropractic and X-ray imaging have come a long way. Today, the chiropractic profession is a leader in the reduction of X-ray exposure and patient safety.