A hierarchical structure is a type of organizational system that arranges items in a hierarchical order, with each item being placed in a specific level. The two most common forms of hierarchical structures are trees and ladders. A tree structure is a branching system where each branch is connected to the one above it, while a LADDER structure is a linear system where each step is connected to the one below it. Both structures are used to organize information or objects in a way that makes them easier to understand and navigate.