The document discusses heap trees, which are tree-based data structures where all nodes follow a specific ordering. There are two main types: max-heaps, where the root node has the greatest value of its children, and min-heaps, where the root node has the minimum value. Heaps are often stored as arrays for efficient implementation of operations like insertion, deletion, and merging in logarithmic time. Common applications of heaps include priority queues, sorting algorithms, and finding order statistics.