Adifference of two is an expression that contains two perfect squares, with one subtracted from the other.

The answer is: "squares".

 1. Therefore, the complete sentence is:

 "A difference of two squares is an expression that contains two perfect squares, with one subtracted from the other".

 2. It has the following form:

 a²-b²

 3. You can factor the expression above as below:

 (a-b)(a+b)

 4. For example:

 x²-4 can be written as x²-2², then it can be factor as following:

 x²-2²=(x-2)(x+2)