An expression tree is a binary tree used to represent mathematical expressions. Each leaf node contains an operand, such as a number or variable, while each internal node represents an operator, like addition or multiplication. This structure allows for easy manipulation and evaluation of expressions. Expression trees are particularly useful in computer science for parsing and evaluating expressions in programming languages. They can simplify complex expressions and facilitate operations like differentiation and optimization. By traversing the tree, one can easily compute the value of the expression or convert it to other forms, such as postfix notation.