Documents may contain collections of references to books, papers, magazines and other type of information, that is references throughout the document. In the referenced section, it's possible to create links to other available documents, if they are available, their URL is know, and known to persist. Also, email-references (mailto:-links) may be inserted to contact an author of a book.
Furthermore, so-called ``backlinks'' can be inserted from the reference of a book back into the places in the document, where this book (or whatever) was references. This makes it easier to find out what for a certain book was being references for. When using the hyperref-macros for a LATEX-to-PDF conversion, this can be done automatically by giving the ``backref''-option when calling the hyperref macros: