Quickselect is an efficient algorithm used to find the k-th smallest or largest element in an unordered list. It works by selecting a pivot element and partitioning the list into two parts: elements less than the pivot and elements greater than the pivot. Depending on the position of the pivot, the algorithm recursively narrows down the search to one of the partitions. This method is similar to Quicksort, but instead of sorting the entire list, it focuses only on the part that contains the desired element. Quickselect has an average time complexity of O(n), making it faster than sorting the entire list when only one element is needed.