You have likely seen diffraction before.  For example if you look at a CD when exposed to white light you can see it diffracted into varies wavelengths of color.  The pits (or grooves) in the CD are the regularity of the material that causes the diffraction.

Google images: outreach.atnf.csiro.au

Diffraction can occur when any electromagnetic radiation interacts with a periodic structure.  The repeat distance of the periodic structure must be about the same wavelength of the radiation.  For example, light can be diffracted by a grating having scribed lines arranged on the order of the wavelength of light.

Question:

So if all electromagnetic radiation can diffract, why are X-rays used in crystallography?