Biological X-ray crystallography is an set of experimental techniques, which allows to obtain the 3-dimensional arrangement of atoms in macromolecules such as proteins. Wikipedia:X-ray crystallography gives a technical overview of what it is about.
One requirement of the technique is to obtain crystals of proteins (or whatever molecule is being studied). Unlike sugar or diamonds, protein crystals usually contain more than 50% of water, which makes the protein more free than what we would think.
Besides the fact that crystals are needed, common criticisms of X-ray crystallography include the fact that unnatural conditions need to be created in order for the protein to crystallize (temperature, pH, ionic concentrations).