The link is the basic building block of hypertext.
The Memex, described by Vannevar Bush in a 1945 Atlantic article "As We May Think" would have allowed associative linking of research materials
Such linking of anything to anything else more closely resembles the workings of the mind than categorization schemes such as alphabetizing. And the Memex was a major inspiration for Ted Nelson's invention of hypertext in 1965.
Links are the visual cues for the various paths a reader may take through a hypertext.
Most readers will have expectations as to where a link will lead, but a literary hypertext may very well confound reader expectations, or fulfill them in unexpected ways.
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.