The document discusses red-black trees, which are binary search trees augmented with node colors to guarantee a height of O(log n). It describes the properties that red-black trees must satisfy, including that every node is red or black, leaves are black, and if a node is red its children are black. It then proves that these properties ensure the height is O(log n) by showing a subtree has at least 2^bh - 1 nodes, where bh is the black-height. Finally, it notes that common operations like search, insert and delete run in O(log n) time on red-black trees.