  The spatial symmetry of matter - including finite objects like molecules or
atomic clusters, and extended objects like periodic or aperiodic crystals - is
described using point groups and space groups. Magnetic point groups and space
groups are the simplest extension of this description, to matter whose atomic
constituents possess a property that can take one of two possible values, like
the "up" or "down" orientations of a magnetic moment, or a spin. Magnetic
groups - also known as antisymmetry groups, Shubnikov groups, Heesch groups,
Opechowski-Guccione groups, as well as dichromatic, 2-color, or simply
black-and-white groups - are here defined, and their structure and notation
explained, while providing some pedagogical examples of their enumeration. The
resulting magnetic selection rules, or extinctions, in neutron diffraction
experiments are discussed. Further extensions to color groups and spin groups
are briefly described.
