Writing a Functional Specification 

A functional specification can substantially simplify and streamline the process of application development. Intended to describe how a piece of software works, it provides a ready reference for software developers andaligns large and disparate development teams to a single goal. In the process, it provides technical clarity on how the different components of aparticular applications are to be designed, implemented and integrated witheach other, and (if used correctly) significantly reduces the time and costcomponent of any development exercise.

Deepa L. Software Reality (2003). Articles>Writing>Specifications>Functional Specifications