Star Clusters fall in two categories: Open Clusters and Globular Clusters.
They consist of an irregular collection of a few dozen to several hundred stars, which are gravitationaly bound. They lie in the disk or spiral arms of our galaxy, the Milky Way, therefore most visible Open Clusters are at a relative short distance. Dust from our galaxy hides Open Clusters farther away. Most open clusters are relatively young, some of the youngest being still within the nebulae in which they formed. It is sometimes impossible to know if the stars from a cluster are gravitationaly bound or if it is an optical illusion (known as "asterism").
They consist of a spherical collection of up to 100,000 stars and can be found both in the disk and halo of the Milky Way. Globular Clusters formed early in galactic history, having ages of 10 billion years or more.