X-rays are a form of electromagnetic radiation (like visible, ultra-violet, etc. light) characterized by wavelengths, l, in the range 1 x 10^-7 to 4 x 10^-10 m :

In crystallography, units of Angstroms (Å) are usually used (1 Å = 1 x 10^-10 m). Another important quantity is the amplitude of a wave - an X-ray experiment generally measures the Intensity of an interaction with X-rays. Intensity is related to the square of the Amplitude.
X-rays are produced in an X-ray tube, an evacuated glass container similar to a cathode ray tube:

A beam of electrons accelerated by an applied voltage of 20 - 50 kV impinges on an anode (usually copper or molybdenum metal).