The term 'document' is changing its meaning. It used to mean printed material, but increasingly we create content online, share or send electronically, and read on screen without ever printing out.
Nevertheless, there is still a need to produce 'paged' material that could potentially be printed out and so includes standard features such as page numbering, tables of contents, images with captions and so on...
Text Processing: a summary of document anatomy and features
Making Documents: the features needed for different document types
Google Docs: tools and features in Google Docs
Word Docs: tools and features in Word documents