The link is the basic building block of hypertext. It is the one thing that hypertext has that other genres do not. Just as most poetry has the line, which allows a poem to deploy words in patterns unavailable to prose writers, hypertext has the link.
The concept of the link is the basis of hypertext. The first hypertextual information system was arguably Vannevar Bush's Memex, described in 1945, but never constructed. This imaginary device would have allowed associative linking of research materials. A researcher, at work at her Memex, could link a photo to a note linked to a page from a book. The article describing the Memex was entitled "As We May Think"; Bush believed that such associative linking more closely resembled the workings of the mind than earlier categorization schemes (such as alphabetizing). And the Memex was a major inspiration for Ted Nelson's invention of hypertext in 1965.
The link thus becomes not only a defining structure of hypertext, but the primary way meanings are made in hypertext. Links are the visual cues for the various paths a reader may take through a hypertext. But links can lead anywhere. In many cases readers will have expectations as to where a link will lead, and in most uses of hypertext, these expectations are borne out (a link on Amazon.com will take me to the page for the correct product unless the system is broken). But a literary hypertext may very well confound reader expectations, or fulfill them in unexpected ways.
This interplay between reader expectation and reader surprise is important in literary hypertext in much the same as it is important in literary fiction: an entirely expected development in a narrative is boring, but an entirely unexpected one may will not be satisfying (discovering that a never-introduced character is the murderer would be an annoying ending to a mystery).
Moulthrop's Hejirascope is an excellent example of this interplay between expectation and frustration.
By using links to connect screens in expected and unexpected ways, hypertext allows readers to explore the text and create meaning.