X- rays have a high frequency and a very short wavelength.
They are used in imaging: A sheet of X ray film is place on one side of a person and X-rays fired at the patient towards the film. If part of an X-ray film is exposed to X-rays (X-rays hit it) then it will darken. The darker it is, the more X-rays have reached it. When X-rays pass through the body, some of them are absorbed. Different types of material absorb different amounts: that's why the bones are whiter than the skin.
X- Rays can be dangerous: too much exposure can lead to cancer. This is why doctors stand behind a lead shield when they are giving you an X-Ray.
X- Rays are also used at airport security checks to look inside luggage.
Because X-rays have such a short wavelength, to see diffraction they need a very small gap. This gap is about the size of the spacing between atoms! This means that we can shine X-rays at crystals and use the diffraction patterns to find out how their atoms are arranged!