Code indicates that a question is to do with pages, controllers or similar, and not the declarative aspects of the platform.

The Force.com/Salesforce.com platform offers two main development methodologies, a traditional style and a declarative (mouse-click) style.

Although seemingly redundant the Code tag does provide a broad-level differentiation point for those looking to only tackle declarative, or code based, questions.

Code based questions could concern 3rd party environments and languages, Apex, VisualForce, HTML, CSS and Javascript among other technologies.