This document discusses 2-3 trees, which are trees where each internal node has either 2 or 3 children and all leaves are at the same depth. Some key rules for 2-3 trees are that internal nodes can have 1 or 2 keys, leaves are all at the same depth, and internal nodes have either 1 key and 2 subtrees or 2 keys and 3 subtrees. The advantages of 2-3 trees are that they may be shorter than binary search trees while still maintaining relative balance simplicity during insertion and deletion.