A star cluster is just what the name says: a clustering or grouping of stars.
These are some of the most beautiful objects to view through binoculars or a telescope. I have found myself getting lost as I stare into the clusters. Some are faint and require averted vision to make out some of the distinct points of lights. Others are large clusters of many bright stars.
I like to look at these objects and think about which clusters are physically grouped together, and which clusters only appear to be close together.
Star clusters come in many different organizations and densities.