These types of search engines use a "spider" or a "crawler" to search the Internet. The crawler digs through individual web pages, pulls out keywords and then adds the pages to the search engine's database. Google and Yahoo are examples of crawler search engines.
The advantage of crawlers are:
There are several disadvantages to crawlers:
Directories are human powered search engines. A website is submitted to the directory and must be approved for inclusion by editorial staff. Open Directory Project and the Internet Public Library are examples of directories.
Hybrids are a mix of crawlers and directories. Sometimes, you have a choice when you search whether to search the Web or a directory. Other times, you may receive both human powered results and crawler results for the same search. In this case, the human results are usually listed first.
Meta search engines are ones that search several other search engines at once and combines the results into one list. While you get more results with meta search engines, the relevancy and quality of the results may sometimes suffer. Dogpile and Clusty are examples of meta search engines.