Several hypertext systems and some WWW-browsers (e.g. NCSA Mosaic, but not Netscape Navigator) let you add annotations to the information nodes. In theory, annotations need not consist of written text, but can be any media. Given the proper hardware you can for instance add audio annotations to this course text, when using NCSA Mosaic (which is not advised for other reasons).
Your annotations are yours only. Several readers can share a hypertext and each add their own annotations without ever seeing annotations made by other readers. Some systems (again, e.g. NCSA Mosaic) have the ability to create annotations which are shared by a group of users.
In general annotations do not become nodes of the hyperdocument. To emphasize this they are normally presented in a somewhat different way, and you cannot add links to annotations or link the same annotation to more than one node. (NCSA Mosaic is an exception however. Annotations in Mosaic look just like normal nodes, and if you know HTML you can even add links to annotations.)