When an expression can be viewed as the difference of two perfect squares, i.e. a²-b², then we can factor it as (a+b)(a-b). For example, x²-25 can be factored as (x+5)(x-5). This method is based on the pattern (a+b)(a-b)=a²-b², which can be verified by expanding the parentheses in (a+b)(a-b).
It is called a difference of squares because there are two things squared being subtracted. You can recognize it when there are two terms, subtraction between the two of them, and you know the square root of both
