A heap is a special tree-based data structure which satisfies the heap properties. The largest key is the root and smaller keys are leaves, which is known as max heap. Or, the smallest key is the root and larger keys are leaves, which is known as min heap. Though the root of a heap structure is either the largest or smallest key of the tree, it is not necessarily a sorted structure. A heap is used for solving graph algorithms with efficiency and also in sorting. We are going to explore heap data structures in Chapter 10, Understanding and Using Heaps.