Due to new technologies in telecommunication, most of computer-based systems are available through Internet or Intranet. Web servers supply with hypermedia tools for user-driven access to information. They provide a new information retrieval mechanism based on browsing into database or information systems instead of writing queries. On-line information systems used to have huge hyperspace and then the user may become hopelessly lost in hyperspace. Users only access a fragment of information space according to their current goal. Consequently, the task model enables us to assist the user for browsing and to determine the relevant information space which corresponds to certain concepts of the domain space. For hyper-document comprehension, coherence and cognitive overheads are the main factors. Domain and task models give local and global guidance to the users in order to convey the overall document structure to them and then to increase the document coherence and decrease the cognitive overheads in enhancing orientation and navigation.