A hypertext system is made of nodes (concepts) and links (relationships). A node usually represents a single concept or idea. It can contain text, graphics, animation, audio, video, images or programs. It can be typed (such as detail, proposition, collection, summary, observation, issue) thereby carrying semantic information [Rao & Turoff, 1990]. Nodes are connected to other nodes by links. The node from which a link originates is called the reference and the node at which a link ends is called the referent. They are also referred to as anchors. The contents of a node are displayed by activating links.
Links connect related concepts or nodes. They can be bidirectional thus facilitating backward traversals. Links can also be typed (such as specification link, elaboration link, membership link, opposition link and others) specifying the nature of relationship [Rao & Turoff, 1990]. Links can be either referential (for cross-referencing purposes) or hierarchical (showing parent-child relationships). Activation of link markers display nodes.