A functional specification describes how a product will work entirely from the user's perspective. It doesn't care how the thing is implemented. It talks about features. It specifies screens, menus, dialogs, and so on - joelonsoftware.com 
# Specification Overview
This is like the table of contents for your spec. It might be a simple flowchart, or it might be an extensive architectural discussion. Everybody will read this to get the big picture, then the details will make more sense.