In a hypertext, the words for which there is more or related information are marked in some visible way. The text may be underlined or appear in a different color (or both). This text is called an anchor. By performing some action, like clicking the left mouse-button while the cursor is positioned over an anchor, you can jump directly to the available additional information. This action is called "following a link". Because the author has indicated the relationship between the anchor (text) and the related information (in the destination of the link), he should have been aware of the possibility to jump from here to there. Therefore, it is much less likely that the related information will be phrased using terminology which you don't understand, since you were not required to read the part of the hyperdocument containing the definitions for that terminology.
When there is more information on a word or technical term that word is not turned into an anchor everywhere it appears in the hyperdocument. Doing so would create a messy abundance of links. The word is only turned into an anchor where the author considers it to be appropriate. So unlike with a written book, the anchors indicate when it may be appropriate to read more about a term which would normally appear in the index.
The connections between nodes, implemented through links and anchors, are not all cross-references. Most hyperdocuments, including this course text, have a partially hierarchical structure. Conceptually, links representing this hierarchical structure are distinguished from cross-reference links that only indicate related information. Usually there is no visual distinction between the two types of links.