The document discusses binary search trees and their variants. It explains that search trees are important for algorithm design and it is desirable to minimize the search time of each node. There are static and dynamic binary search trees, with the latter adjusting its structure during access. AVL trees are a type of self-balancing binary search tree where rotations are used to rebalance the tree after insertions or deletions and ensure the heights of subtrees differ by at most one. Compilers use symbol tables implemented as search trees to track variables in source code.