The document discusses 2-3 trees, which are balanced search trees invented by John Hopcroft in 1970. 2-3 trees have the following properties: each node contains 1 or 2 keys, each internal node has 2 children if it has 1 key or 3 children if it has 2 keys, and all leaves are on the same level. The document covers insertion, search, and removal algorithms for 2-3 trees. Insertion may require splitting nodes and promoting keys up the tree to maintain the balanced structure. Search works by recursively traversing the tree to the appropriate child node based on key comparison. Removal is the reverse of insertion, removing nodes by restructuring child pointers or replacing keys.