Star clusters are groups of stars. There are two types of star clusters. Globular clusters are tight groups of hundreds or thousands of very old stars which are gravitationally bound and Open clusters which are more loosely clustered groups of stars, generally they contain fewer than a few hundred members, and are often very young.
Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the galaxy, but cluster members will continue to move in broadly the same direction through space even though they are no longer gravitationally bound; they are then known as a stellar association, sometimes also referred to as a moving group.
More than 150 globular clusters were known in the Milky Way Galaxy by the early years of the 21st century. Most are widely scattered in galactic latitude, but about a third of them are concentrated around the galactic centre, as satellite systems in the rich Sagittarius-Scorpius star fields.