This document discusses the stack data structure. It defines a stack as a linear data structure where elements are added and removed from only one end, called the top. Common stack operations like push, pop, and peek are described. Examples are given to illustrate how stacks can be used to perform calculations. The document also explains how stacks can be implemented using both arrays and linked lists. Key points are that arrays maintain a size and add/remove from the first element, while linked lists use a head node and insert/remove as the new head.