Scaffold is a toolkit for building web applications.
Web application frameworks, like Struts, provide the essential infrastructure most developers need to create applications. But there are still many reusable pieces that most applications need but frameworks don't typically provide. Scaffold is a home for classes that can be reused between applications but are not sexy enough (or sometimes too sexy) for a framework distribution.
The bulk of Scaffold is being provided through the Commons. A Struts specific package is provided in the Struts source code distribution which builds on the classes provided here.
The JavaDoc API documents are available online.