The document discusses AVL trees and balanced binary search trees. It provides the following key points: 1) An AVL tree is a self-balancing binary search tree where the height of the two child subtrees of every node differ by at most one. 2) A balanced binary search tree is one where the height of the left and right subtrees of each node differ by no more than one. 3) Inserting new nodes can cause the tree to become unbalanced if the insertion breaks the height balance property. Rotations may be needed to rebalance the tree.