Stars do not only occur as " individuals" but also in physically connected groups. There are two different main types of clusters: open clusters and globular clusters.
Open Clusters consist of young stars, at most a few hundred million years old. These were formed together in a molecular cloud and move through space at a similar speed. The number of stars within an open cluster is relatively low. The larger ones contain just a few hundred of them. Therefore the gravitational binding between the stars is relatively weak and they dissolve gradually over a long period of time.
Globular Clusters belong to the oldest objects in the universe and are several billion years old. Some of them contain more than one million stars, which hold the cluster in their spherical shape by their gravitational forces. In contrast to the open clusters, the globular clusters do not only move within the galactic plane but form a spherical halo around our galaxy. Almost 160 globular clusters are known around our Milky Way.