Example of web indexes are Web Crawlers and Altavista. Web directories, on the other hand, are better organized and informative. With the help of human aids, these programs can produce better result and less search time. However, the search list is not as exhaustive as web indexes, and the search results can be subjected to advertising effect, which can put a question of the integrity of the search result. Examples of web directories are Yahoo and Lycos.
Web catalogs are database that are compiled from the web and stored on the web site.
Users need only to log on to the web catalog and search for it. Weak points are lacking of a real search engine and information might be dated. Offline databases are compiled and stored offline on CD-ROM or other media. Weak points are the same as web catalog. Yahoo search is a typical search engine.