A disjoint-set is a data structure that keeps track of a collection of non-overlapping sets. It supports two primary operations: union, which merges two sets, and find, which identifies the set containing a specific element. This structure is useful in various applications, such as network connectivity and clustering. Disjoint-set is often implemented using techniques like path compression and union by rank to optimize performance. These optimizations help ensure that the operations can be performed in nearly constant time, making disjoint-set efficient for large datasets and complex problems, such as those found in graph theory and Kruskal's algorithm.