An open cluster is a type of star cluster made of tens to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age.
In spiral galaxies, open clusters are largely found in the spiral arms where gas densities are highest and so most star formation occurs, and clusters usually disperse before they have had time to travel beyond their spiral arm.
In irregular galaxies, open clusters may be found throughout the galaxy, although their concentration is highest where the gas density is highest.
Open clusters are not seen in elliptical galaxies: Star formation ceased many millions of years ago in ellipticals, and so the open clusters which were originally present have long since dispersed.