A cluster of stars is called a globular cluster, a super star cluster, an embedded cluster or an open cluster. There are also extended globular clusters, unique to only the Andromeda Galaxy, as far as scientists know, as of 2014.
A globular cluster ranges from thousands to millions of stars, spreading from 10 to 30 light-years across. Globular clusters are compact and contain the oldest star formations. A super star cluster is a large formation of stars that scientists believe foreshadows globular clusters. An embedded cluster is uniquely identifiable, as they are surrounded by gas and dust. Open clusters are generally newer formations, containing only hundreds of stars across a 30-light-year distance.