Star clusters are groups of stars that are held together by gravity. They can be classified into two main types: open clusters and globular clusters. Open clusters, like the Pleiades, contain a few dozen to a few thousand stars and are often found in the Milky Way galaxy. They are typically younger and have a more scattered appearance. Globular clusters, on the other hand, are densely packed groups of stars, often containing hundreds of thousands of stars. They are usually older and have a spherical shape. An example of a globular cluster is M13, located in the constellation Hercules. These clusters provide valuable insights into the formation and evolution of stars.