Can someone please explain what a "Tagged PDF" is, and how it differs from regular, non-tagged PDF?
Will tagged PDFs contain special content, such as XML, Rich Media, Javascript, or the like?
Which TeX-toolchains generate Tagged PDFs?
Tagged PDF is a PDF file that contains meta-information around certain groups of PDF instructions inside a page content. This meta-information has many use cases: Text-extraction, content-reflow, document accessibility, geographic information in PDF containing maps, etc.
If you need to know more details about this topic I would recommend reading Chapter 10 - Document Interchange of Adobe PDF Reference version 1.7.