A group of galaxies which are mutually gravitationally bound. Two important types of galaxy cluster are:
Regular Galaxy Cluster: These are great groupings of galaxies into huge spherical distributions that have large numbers of galaxies concentrated in their centers. The tend to contain thousands of galaxies and to have many bright elliptical and S0 type galaxies.
Irregular Galaxy Cluster: Clusters of galaxies that are not too centrally condensed, with a somewhat nonspherical overall shape, containing a few galaxies up to hundreds of galaxies. Our local group is an example of an irregular cluster of galaxies. Clusters of galaxies contain all types of galaxies; despite the name, this type of cluster contains more than "irregular galaxies."