X-rays are a form of electromagnetic radiation with a wavelength approximately in the range of 0.5 pm - 10 nm (corresponding to frequencies in the range 30 PHz - 600 EHz).
X-rays with a wavelength longer than 0.1 nm are called soft X-rays. At wavelengths shorter than this, they are called hard X-rays. Hard X-rays overlap the range of long-wavlength (low energy) gamma rays, however the distinction between the two terms refers to the source of the radiation, not its wavelength: X-ray photons are generated by energetic electron processes, gamma rays by transitions within atomic nuclei.
X-rays were first observed and documented in 1895 by Wilhelm Conrad Roentgen, a German scientist who found them quite by accident when experimenting with vacuum tubes. A week later, he took an X-ray photograph of his wife's hand which clearly revealed her wedding ring and her bones. The photograph electrified the general public and aroused great scientific interest in the new form of radiation. Roentgen called it "X" to indicate it was an unknown type of radiation. The name stuck, although (over Roentgen's objections), many of his colleagues suggested calling them Roentgen rays. They are still occasionally referred to as Roentgen rays in German-speaking countries.
X-rays are highly penetrating of many materials, and are used in medicine to take pictures of bones and teeth. This is because bones absorb the radiation more than the less-dense soft tissue. X-rays from a source are passed through the body and onto a photographic plate; areas where radiation is absorbed show up as white and can be used to show broken or fractured bones.
Exposure to x-ray radiation is dangerous and cancer causing, so radiographers try to keep exposure to the patient to a mimimum. Radiographers themselves wear lead aprons or stay behind a lead screen.
X-ray Astronomy is the study of the universe by analyzing the x-ray radiation which we receive on the earth.
Some lasers rely on x-rays.