If you've used any sort of basic online help system, you're already familiar with the primary concept behind the World Wide Web: hypertext. The idea behind hypertext is that instead of reading text in a rigid, linear structure (such as a book), you can skip easily from one point to another. You can get more information, go back, jump to other topics, and navigate through the text based on what interests you at the time.
Online help systems or help stacks such as those provided by Microsoft Windows Help, or help stacks such as those provided by HyperCard on the Macintosh, use hypertext to present information. To get more information on a topic, you just click on that topic. The topic might be a link that takes you to a new screen (or window or dialog box) that contains the new information. Perhaps you'll find links on words or phrases that take you to still other screens, and links on those screens that take you even further away from your original topic.