Idea— Imagine a hand of playing cards: you hold already-sorted cards on the left, then pick a new card (the key) from the right. You shift cards to the right until the correct gap is found, then insert the key. The sorted region grows by one card.

Complexity — O(n²) in the worst case (reverse-sorted array), O(n) in the best case (already sorted — only comparisons, no shifts). Better than Bubble Sort in practice for small arrays.