What Is xref?
When included in a Web page, xref will examine the text of that page, looking for predefined key terms within it. When such a key term is found, the script automatically converts that term into a clickable link, with the link itself also going to a pre-defined URL (a page that further describes the term, or is somehow directly related to the term in question). A predefined title will also be applied to the term (which most modern browsers will display when the user hovers over the link), if so desired.
As an example, let's say that we operate a site that focuses on healthy eating. Within our site, we have several reference pages that are each dedicated to a specific type of food, describing their long-term benefits or harmful effects. It would be beneficial to us, from a traffic building standpoint, to be able to include links on all of our pages back to the reference pages whenever these foods are mentioned in our posts. Such links would also be helpful to our visitors, since the pages they point to will provide additional information pertaining to the food that may not be immediately available within the context of the page they're reading.
xref allows you to define those reference terms and links in a single place--namely, the xref script-- then include the script in your pages such that it will automatically insert the links each time a visitor reads the page. Here's a live example, using links to Wikipedia for several selected fruits:
Note the links to the fruits in the above brief paragraph. The important thing to realize it that those links were not included in the original code of this page (feel free to view the source code to verify this for yourself if necessary); but were instead inserted automatically by xref. You can therefore include xref on all your pages and keep your reference links up to date automatically!
Who Needs xref?
Typical users of xref will be those with pages that deal in some specific way with targeted terms. Reference sites are easy examples, of course; since the terms or topics defined by your reference pages can easily be linked to and provide value-added information to site visitors. But other types of sites might also benefit; such as E-commerce sites (links to products you are selling), or even a site that offers free cooking recipes. Use your imagination! And remember, you can provide the script as a free service to all of your affiliates, as well.
What xref Isn't
Though it provides multiple advantages to Webmasters, xref is not entirely without limitations. Key deficits of xref that you will want to consider before implementing include: