What is X-ray Crystallography ?
X-ray crystallography is an experimental technique that exploits the fact that X-rays are diffracted by crystals. It is not an imaging technique. X-rays have the proper wavelength (in the Ångström range, ~10-8 cm) to be scattered by the electron cloud of an atom of comparable size. Based on the diffraction pattern obtained from X-ray scattering off the periodic assembly of molecules or atoms in the crystal, the electron density can be reconstructed. Additional phase information must be extracted either from the diffraction data or from supplementing diffraction experiments to complete the reconstruction (the phase problem in crystallography). A model is then progressively built into the experimental electron density, refined against the data and the result is a quite accurate molecular structure.
Why Crystallography ?
The knowledge of accurate molecular structures is a prerequisite for rational drug design and for structure based functional studies to aid the development of effective therapeutic agents and drugs. Crystallography can reliably provide the answer to many structure related questions, from global folds to atomic details of bonding. In contrast to NMR (which is a spectroscopic method), no size limitation exists for the molecule or complex to be studied. The price for the high accuracy of crystallographic structures is that a good crystal must be found, and that limited information about the molecule's dynamic behavior in solution is available from one single diffraction experiment. In the core regions of the molecules, X-ray and NMR structures agree very well, and enzymes maintain their activity even in crystals, which often requires the design of non-reactive substrates to study enzyme mechanisms.
You can read the first pages 1, 2, 3 of the introduction of my book Biomolecular Crystallography or buy the book from Amazon.