Be sure to try out a handful of sites when researching a topic on the Internet. Do not rely on only one site or one type of site. Directory services use selection criteria for choosing links to include, though the selectivity varies among services. Most directories offer a search engine mechanism to query the service. Be sure you use the directory that appropriately meets your needs.
Subject directories differ significantly in selectivity.
Consider the policies of any directory that you visit. One challenge to this is the fact that not all directory services are willing to disclose either their policies or the names and qualifications of site reviewers. This is especially true of commercial portals. Many people don't make enough use of subject directories, but instead go straight to search engines. Keep in mind that academic subject directories contain carefully chosen and annotated lists of quality Internet sites.