Sentence:  Together, the MathDocument and PaperController classes comprise the MathPaper application's front end, while the Evaluator makes up the application's <WSD>back end</WSD>. This technique of having separate front and <WSD>back ends</WSD> is a common ... 
Meaning of the word marked with <WSD> tags:  That part of a hardware or software system that is farthest from the user; the internals rather than the user interface.