These markers indicate that someone has made an annotation on that particular phrase of text, which you can view by selecting either of the two markers. It is important to realize that the author of the annotation did not need to modify the document you were reading to allow others to make annotations. If your browser does not support this attribute, enabling JavaScript will make this information appear in the status bar of your browser. This brings up a form where you can enter your comment. You specify the phrase to annotate by copying and pasting the phrase into the form.
Any web document can annotate any other document.
So if you have a web site where you can place your own documents, you can publish annotations yourself. Once someone views your annotation document through the mediator, your links will be recorded so that your annotation will appear in the list of backward links at the end of the target document. To annotate a specific phrase, you need to use a little bit of special syntax. Simply take the URL of the target document and append a hash mark followed by a single quote and the sequence of words in the target. The words should be joined with hyphens and the target phrase should be pointed out with parentheses.