What is the key difference between a galaxy cluster and a supercluster?

Clusters are gravitationally bound, superclusters often are not

The distinction between a galaxy cluster and a supercluster lies fundamentally in their gravitational binding status and structure. A galaxy cluster is defined as a colossal, gravitationally bound structure containing hundreds to thousands of galaxies, destined to survive intact throughout cosmic time. In contrast, a supercluster is a much larger, looser association of multiple galaxy clusters and groups. While superclusters cover vast regions where density is slightly higher than average, they are generally not gravitationally cohesive structures that will remain bound together as a single unit over the age of the universe; their constituents may eventually separate or fall into even larger structures.