As the name implies, these search sites have databases which are arranged in subject hierarchies, to facilitate browsing through the entries. This approach provides a very user-friendly, easily understood approach to finding things in the database, but becomes cumbersome as the size of the database grows. These sites will usually also have a search' feature to find key words or phrases wherever they appear in the hierarchy. Advantages Disadvantages Similar types of websites grouped together.
Human intercession provides a level of filtering and quality control.
Human-labor intensive process limits database currency, size and rate of growth. Not practical for very large databases. Subject designations may be arbitrary, confusing or wrong. Rather than relying on human beings to discover new web sites, and add them to the searchable database, these sites utilize automated web-scouring software that algorithmically follow links on the web to discover as many Internet sites as possible, index them, and add them to the database. This allows for much more complete coverage of all the sites out there.