Lists of stars and deep sky objects, i.e., M11 (Messier Object No. 11); NGC (New General Catalog); IC (Index Catalog); SAO (Smithsonian Astrophysical Observatory); GCVS (General Catalog of Variable Stars)
Directories provide a structured listing of registered web sites in different categories. They are similar to an electronic version of the Yellow Pages. Yahoo and Excite are the best known examples of directories. The distinction between search engines and catalogues has become blurred since many sites now include both facilities as part of a Portal service.