In essence, hypertext can be described as these two types: "chunks of textual information" and "links" (references) between these chunks. Think of a textual chunk as a building in a city. To go from one building to another, you use streets. Links are the streets of hypertext—you use them to go from one chunk of text to another.
It doesn't seem much, does it? In fact, according to this definition, any book or newspaper article will easily qualify as a hypertext system. After all, you can say that in any book: its paragraphs, its sections, its chapters, and its parts are all linked to one another. They follow each other, explicitly (on the book pages), and implicitly through the plot of the book (introduction, pro and con arguments, and conclusion). Most of the time, you will find references within the text to footnote explanations and figures or tables. So, what's the difference?
The major difference is that books or articles are linear (also called sequential or serial). When you read a book, there is one line or one sequence to follow—and it never changes. A book has only one plot. While in a hypertext document, you can jump anywhere in the text by following a link.