There are three Search Engine types:

1.   Crawler-Based Search Engines: Crawler-based search engines use automated software machines called robots to visit, read and index pages on websites. All of these billions of pages are stored on a large server. So billions of pages are stored and when you type in a subject into the search box, the results are pulled from the large server and delivered to your screen. Crawler-based search engines (like Google) continuously revisit websites and re-index any new information. You want the search engines to continuously re-index your site because it helps improve your rank.

2.   Human Powered Search Engines: Human powered search engines are created by human submitted web pages. Sometimes these are called directories. You might be surprised to know that Yahoo! started out as a human powered search engine or directory. Now though, it is not human controlled. Having a link from a directory like Yahoo! incoming to your site is good because it is a very popular directory and this helps improve your rank. Human powered search engines may require a human to approve the site and therefore take much longer to come up in the results than the crawler-based search engines.
3.  Hybrid Engine: A Hybrid Engine is a combination of a human powered search engine and a crawler based search engine. Websites can be manually submitted for inclusion to the search results and there is a web crawler that monitors the web for sites to include.