Quicksort is an algorithm that uses divide and conquer to sort an array. It works by partitioning the array around a pivot element, placing all elements less than the pivot before it and all elements greater than it after. The key process is partition(), which puts the pivot in its correct sorted position and groups smaller elements left and larger right. Quicksort has average time complexity of O(n log n) but can be O(n^2) in the worst case.