What is a cluster of galaxies?
Galaxy clusters are gravitationally bound system of galaxies with anywhere between 100 galaxies to a 1000 galaxies.
Matter in the Universe is not distributed uniformly on small scales. The clump into stars which agglomerate to form galaxies. The galaxies themselves clump to form larger structures.
Depending on the size of the structures they are named as follows.
Galaxy Groups : An agglomeration of few dozen galaxies,
Galaxy Clusters : An agglomeration of a few hundred galaxies,
Galaxy Superclusters: An agglomeration of thousands of galaxies.