A galaxy is an assembly of stars. The mass of a galaxy is an order of magnitude greater than one hundred million solar masses. Galaxies interact with their neighbors and these interactions change their shape. Two or more galaxies sometimes merge. The galaxies are of three types: elliptical, spiral, irregular.
Like the stars, which are grouped into galaxies, most galaxies are gravitationally bound together.
The structure that holds up to fifty galaxies is called group of galaxies. The structure containing thousands of galaxies grouped in a few megaparsecs sector is a cluster of galaxies.
Groups and clusters of galaxies are themselves grouped into superclusters, giant collections containing tens of thousands of galaxies.
In very large scale distribution of galaxy clusters is not uniform, but organized in sheets or filaments.