The document discusses stacks, which are linear data structures that only allow insertion and removal of elements from one end. Stacks follow the LIFO (last in, first out) principle. The basic stack operations are push, which adds an element to the top, and pop, which removes the top element. Stacks can overflow if full and underflow if empty. Stacks can be implemented using arrays, with a pointer tracking the top index. Pseudocode provides algorithms for push and pop operations on a stack implemented using an array.