X-rays are a form of invisible, high-frequency electromagnetic radiation. The  wavelength of x-rays is between 10 and 0.01 nanometers, corresponding to a  frequency of 30 PHz to 30 EHz. X-rays are very small and energetic. They are  produced by accelerating electrons at a metal target. X-rays are used in various  medical applications, especially for imaging.

Because of the tiny wavelength of x-rays, they are useful for imaging small structures, such as individual molecules. This research field is known as x-ray crystallography, which was responsible for uncovering the physical structure of DNA, among other achievements. Today it is frequently used to image the molecular structure of complex proteins.
Sometimes x-ray imaging is used to investigate paintings which have been painted over. Their most prominent use is probably to image bones to discover whether they are actually broken or not. X-rays are a carcinogen, so use of them for medical purposes should be kept brief.