The document discusses stacks and their applications. It provides 3 key points: 1. A stack is an abstract data type that follows LIFO (last-in, first-out) principles with push and pop operations. Functions like stack_full and stack_empty are used to implement stacks. 2. Stacks have applications in converting infix notation to postfix notation and evaluating postfix expressions. The algorithms pop and push operators and operands to convert between notations and perform calculations. 3. Examples show converting the infix expression (A * B + (C - D / E)) to postfix and evaluating the postfix expression 50, 60, +, 20, 10, -, *.