The bubble sort.
This algorithm is not very popular, however it is still useful. The bubble sort compares adjacent items and swaps them if they are out of order. During each pass, until the array is sorted, the algorithm traverses through the data and compares adjacent items, swapping them if they are not in ascending order. It is simple to understand and implement. However, this sort requires several passes over the data, and thus introduces a major factor of inefficiency.