The first type of star cluster is the open cluster. Open clusters have hundreds or even thousands of stars, and, as evidenced by their main sequence turnoff magnitude, they are all fairly young. This makes sense. When stars are born, they each some have random motion of their own, and, over time, they will wander away from the stellar nursery. The other type of galactic star cluster is the globular cluster.
Globular clusters contain hundreds of thousands, or even millions of stars.
There are so many stars in a globular cluster that the random motions of the stars within them is insufficient to overcome their mutual gravitational attraction. This type of cluster will not disintegrate over time, but will stay together virtually forever. The extreme age of globular clusters allows us to test a prediction of stellar evolution. Recall that each generation of stars is made out of the material lost by previous generations of stars. Recall also that, with each generation, some supernovae go off, ejecting newly formed heavy elements into the interstellar medium.