A globular cluster is a spherical collection of many tens of thousands of stars that are tightly bound (tied) together by gravity.
The stars are all orbiting around the middle of the cluster, and often remind people of bees swarming around their hive.
Globular clusters are found in a large sphere (called the halo) around spiral galaxies, but are located well away from the spiral arms of the galaxies they surround. They are also found in the outer reaches of large elliptical galaxies.
Our own galaxy, the Milky Way is known to have around 150 globular clusters.