Symmetry and space groups
Space group operators can be found here: http://www.embl-hamburg.de/~pinotsis/spacegroup.html
Facts about space groups
There are 230 space groups, 65 of which accessible to proteins.
These are explained and listed in Section 2.2.13 of International Tables of Crystallography, Volume A . If this is not on your desk, check out the IUCr wiki:
Reflection_conditions which explains the topic, and links to lists of the general absences in article integral reflection conditions, and the absences along the screw axes in serial reflection conditions.