What must be true for an expression to be a difference of two squares?

For an expression to be a difference of two squares, it must be in the form (a^2 - b^2), where both (a) and (b) are real numbers or algebraic expressions. This means that the first term must be a perfect square, and the second term must also be a perfect square. The expression can then be factored as ((a - b)(a + b)).