Star clusters are large groups of stars relatively close to one another, appearing as concentrations or dense swarms of stars. Only a couple of them can be seen by the naked eye, most notably including the Pleiades (M45) and the Beehive Cluster (M44).

There are two types of star clusters:

Globular clusters are so tightly packed that they form a globe bound together by gravity and usually slowly rotating (slower than galaxies).  Several notable examples include M4, M5, M13 and M15. The individual stars in the globular clusters have their own movement within the cluster. NASA’s Hubble Space Telescope discovered a lot of orphaned globular clusters that are not part of any galaxy.

Open clusters are loosely bound and are often not gravitationally bound, or ceased to be sometimes in the history of its development, but continue to broadly occupy the same region (i.e, have similar velocities relative to where they are in a galaxy (those that we observe are in the Milky Way). Open clusters usually contain less than few hundred stars. Often, they are part of a nebulosity surrounding it, such as Melotte 15 within the Heart Nebula, for instance.