Regions are two-dimensional filters that can be used to include or exclude data from a given data file. They may be applied to any column in a file (time, spatial, etc.). Regions can be read and written by the ds9 imaging tool and read by CIAO tools (e.g. using a region with dmextract to extract just the source photons).
Regions are made up of a number of shapes that are listed in the dmregions ahelp page. The shapes can either be inclusive or exclusive and can be combined using either a boolean AND or boolean OR operation.
For more information on creating and using regions, see the following threads: