Star Clusters
Star clusters are groups of stars held together by gravity. Open star clusters form in stellar nurseries within the disk of the Galaxy. Globular clusters are composed of older population stars and exist in the Galactic halo, the spherical component of the Milky Way where older, dimmer stars exist.
OB Associations are groupings of stars that have moved apart over time and are no longer gravitationally bound. These formed together and contain younger stars, but have had enough time for its member stars to drift apart.