You should know the diffraction condition $\Delta\vec{k}=\vec{G}$ and be able to explain how the reciprocal lattice vectors can be determined in an experiment.
Given the wavelength of the x-rays, you should be able to tell which diffraction peaks will occur in an experiment. This can be determined with the Ewald sphere.
You should be able to explain how the Bravais lattice and the size of the unit cell can be determined from the measured reciprocal lattice vectors.
You should know how to calculate structure factors from the atomic form factors and know that the square of the structure factors is proportional to the intensity of the diffraction peaks.
You should know how the intensity of the diffraction peaks can be used to determine the basis of the crystal structure. (The basis is the pattern of atoms that are repeated at every Bravais lattice site to create the crystal.)
Diffraction will occur if the incoming $\vec{k}$ vector falls on a Brillouin zone boundary. You should know how to construct the Brillouin zones of a crystal.
The Bravais lattice points always fall on netplanes. The distance between two adjacent netplanes is $d_{hkl}= \frac{2\pi}{|\vec{G_{hkl}}|}$. You should be able to define netplanes and lattice planes. Lattice planes are specified by coprime integers $h$, $k$, and $l$, which are the same as the Miller indices.
Diffraction can be explained in terms of reflections from lattice planes. In this interpretation, the diffraction condition can be written $n\lambda = 2d_{hkl}\sin\theta$ where $h$, $k$, and $l$ are coprime, $\lambda$ is the wavelength and $n$ is a nonnegative integer.
You should be able to explain: powder diffraction, electron diffraction, LEED, and neutron diffraction.
