Unlike the typical printed book, which is read sequentially from beginning to end, hypertext is inherently nonlinear: it is comprised of many interlinked chunks of self-contained text. Readers are not bound to a particular sequence, but can browse through information intuitively by association, following their interests by following a highlighted keyword or phrase in one piece of text to bring up another, associated piece of text. Figure 3.1 illustrates this difference.
The word self-contained is important. Whereas in traditional, linear writing, a piece of text has a well-defined context and is embeded within the linear structure of the work, in a hypertext environment a particular piece of text may be reached from any number of contexts, that is, other chunks of text. Hence it becomes important to avoid assumptions of prior knowledge and keep individual pieces of text as self-contained as possible.
Hypermedia is the generalization of hypertext to include other kinds of media: images, audio clips and video clips are typically supported in addition to text. Individual chunks of information are usually referred to as documents or nodes, and the connections between them as links or hyperlinks the so-called node-link hypermedia model. The entire set of nodes and links forms a graph network. A distinct set of nodes and links which constitutes a logical entity or work is called a hyperdocument; a distinct subset of hyperlinks is often called a hyperweb.
A source anchor is the starting point of a hyperlink and specifies the part of a document from which an outgoing link can be activated. Typically, the user is given visual cues as to where source anchors are located in a document (for example, a highlighted phrase in a text document). A destination anchor is the endpoint of a hyperlink and determines what part of a document should be on view upon arrival at that node (for example, a text might be scrolled to a specific paragraph). Often, an entire document is specified as the destination and viewing commences at some default location within the document (for example, the start of a text). Figure 3.2 illustrates these concepts graphically.
Some authors distinguish between referential and organizational hyperlinks. Referential links are the cross-references distinctive of hypermedia. Organizational links are special links which establish explicit structure by connecting a parent node with its children, forming a tree within the overall node-link graph.
The traditional definition of hypermedia as being `multimedia with links' belies many of the possibilities modern technology now offers. We like to define `real hypermedia' in a broader sense, with two additional components. Firstly, real hypermedia incorporates new technologies like interactive movies, panoramic images, navigable three-dimensional models, and virtual reality. Secondly, real hypermedia involves more than read-only browsing: it possesses integral facilities for communication and collaboration such as annotations, structured discussion, user feedback, message passing, and collaborative authoring.