The expression x² + 1 is not a difference of squares because it cannot be factored into the form a² - b², which requires a real number b. Instead, it can be expressed as x² - (-1), leading to complex factors (x - i)(x + i). In contrast, x² - 1 is a difference of squares, as it can be factored into (x - 1)(x + 1) using real numbers. The distinction lies in the nature of the terms involved; x² + 1 involves imaginary numbers, while x² - 1 remains within the realm of real numbers. Understanding these differences is crucial for proper factorization in algebra.