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, making it useful in compilers and calculators. Expression trees can be constructed from infix, prefix, or postfix notation. They facilitate operations like simplifying expressions, evaluating them, or converting between different notations. By breaking down complex expressions into a tree format, it becomes easier to analyze and compute their values systematically.