Shell sort is a sorting algorithm developed by Donald Shell in 1959. It is an extension of the insertion sort algorithm and is classified as an in-place comparison sort. Shell sort aims to improve the performance of insertion sort by sorting elements that are far apart from each other first, thus reducing the number of comparisons needed to sort the entire array. The algorithm works by first dividing the array into smaller subarrays, each containing elements that are a certain gap apart. The gap is typically initialized to be the length of the array divided by 2, and is then halved in each iteration until it becomes 1. The subarrays are then sorted using insertion sort, which sorts elements within each subarray by comparing adjacent elements and swapping them if they are in the wrong order. The key idea behind Shell sort is that insertion sort performs poorly on arrays that are almost sorted, but works well on small arrays. By sorting the subarrays with larger gaps first, the algorithm