What is the difference between stable and unstable sorting algorithms?

A stable sort preserves the relative order of elements with equal keys - if two elements are equal, they appear in the same order in the sorted output as in the input. Unstable sorts may change this relative order. Stable sorts: Merge sort, Insertion sort, Bubble sort. Unstable sorts: Quick sort, Heap sort, Selection sort. Stability matters when sorting by multiple fields or when preserving original order for ties is important.