Galaxy superclusters are among the largest structures in the universe. NASA describes them as large collections of galaxy clusters, galaxy groups, and individual galaxies that are typically not gravitationally bound to each other, so unlike a single galaxy cluster they are not held together as one unit.
Our own Milky Way belongs to the Local Group of galaxies, which sits on the outer edge of the Laniakea supercluster. Superclusters in turn line up into walls or sheets of galaxies and form part of the cosmic web, the network of threadlike structures that spans the observable universe.
Source: NASA Science: Large Scale Structures
Image: galaxy cluster Abell 2261, NASA, ESA (Hubble Space Telescope), public domain, via Wikimedia Commons.