Using all of the non-translational and translational symmetry elements, a total of 230 unique space groups can be defined. Information for each of these space groups is contained in a volume titled “International Tables for X-ray Crystallography”. The page for space group is reproduced below:



Key:

1The asymmetric unit is the smallest piece of structural information needed to reproduce the entire three-dimensional structure of the crystal. Often, the asymmetric unit is the same as one molecule, but this isn't a requirement.
2The symmetry related positions are the atom locations produced by the various symmetry operations. In this space group, for an atom at general location (x, y, z), there is an identical atom at (-x, -y, -z).
3Special positions occur when an atom lies on a symmetry element. In this space group (as in all centrosymmetric unit cells), there are eight centers of symmetry. If an asymmetric unit lies on one of these centers, there is no requirement that it lie on all of them, only that they all are centers of symmetry. For an example of a molecule that lies on a special position, go here.