This document discusses expressions, equations, variables, and order of operations in algebra. It defines an expression as a number or variable combined with operations like addition and multiplication. An equation sets two expressions equal to each other. Variables represent unknown numbers. The order of operations for evaluating expressions is: 1) operations in parentheses, brackets, and braces; 2) exponents; 3) multiplication and division from left to right; 4) addition and subtraction from left to right. Examples show how to evaluate expressions by plugging in values for variables and following the proper order.