The general form for difference of squares expressions is:

a^2 - b^2

Since two terms are being subtracted (which is called a difference) and they're both squared, it's given the name "difference of squares."

The general form of the factors in this case will look like:

(a + b)*(a - b)

If you FOIL these two binomials (two-term factors) by multiplying them together, you get:

a*a - a*b + a*b + b*b = a^2 - b^2

Which is your original expression. This proves that the factor we showed for the expression is correct.