Directory services use selection criteria for choosing links to include, though the selectivity varies among services.
Most directories are searchable.
Academic and professional directories are created by librarians or subject experts and tend to be associated with libraries and academic institutions. These collections are created in order to enhance the research process and help users find high quality sites of interest. A careful selection process is applied, and links to the selected resources are usually annotated. These collections are often created to serve an institution's constituency but may be useful to any researcher. As a rule, these sites do not generate income or carry advertising.