The Visual Layer, also referred to as the Physical Layer, is the layer that produces the "look" of the PDF file. It would mirror the document if printed to a hard copy. It is the layer you see when viewing a PDF file.
The Content Layer, also referred to as the Text Layer, contains the textual information that is associated with the Visual Layer. Although you cannot view this layer, its existence allows users to search the document or make minor editing changes. By default, the text layer will follow a linear top-bottom, left-right reading order. The content layer is text only and does not contain any formatting structure such as headings, list bullets, or specialized markup.
The Tags Layer contains the PDF tagging structure (similar to HTML markup for a webpage) that is used by Assistive Technology. Tag markup allows Assistive Technology to differentiate between elements such as headings, paragraphs, lists, tables, and forms. Without a tagging structure, a document will appear blank to many Assistive Technology devices. The reading order of the document can also be edited for Assistive Technology devices through the Tagging order.