Term 'search engine' is generally used to describe search engines based on crawling technology and human edited web directories. Both the search engines operates differently.

Search Engines Based on Crawling Technology

Search engines like Google is considered as Crawler based search engines and they build their listings automatically. They "crawl" or "spider" the web pages, then people search through what they have found.

If you change your web pages, search engines operates on crawling technology, eventually find these changes automatically. They will rate your website based on the changes you have made and that can affect how you are listed. Page titles, meta tags, header, body and all other elements plays the major role in your website listing.

Human Edited Web Directories

A human edited web directory, such as DMOZ - Open Directory, depends on humans for its listings. You submit a link with short description about your website to the directory, human editors review your website and list you on the directory. A search looks for matches only in the descriptions submitted.

Changing your web pages has no effect on your listing. Things that are useful for improving a listing with a search engine have nothing to do with improving a listing in a directory. The only exception is that a good site, with good content, might be more likely to get reviewed for free than a poor site.

Hybrid Search Engines

In the web's early days, it used to be that a search engine either presented crawling based results or human edited listings. Today, it extremely common for both types of results to be presented. Usually, a hybrid search engine will favor one type of listings over another. For example, MSN Search is more likely to present human-powered listings from LookSmart. However, it does also present crawler-based results (as provided by Inktomi), especially for more obscure queries.

The Parts Of A Crawling Based Search Engines

Crawler-based search engines have three major elements. First is the spider, also called the crawler. The spider visits a web page, reads it, and then follows links to other pages within the site. This is what it means when someone refers to a site being "spidered" or "crawled." The spider returns to the site on a regular basis, such as every month or two, to look for changes.

Everything the spider finds goes into the second part of the search engine, the index. The index, sometimes called the catalog, is like a giant book containing a copy of every web page that the spider finds. If a web page changes, then this book is updated with new information.

Sometimes it can take a while for new pages or changes that the spider finds to be added to the index. Thus, a web page may have been "spidered" but not yet "indexed." Until it is indexed -- added to the index -- it is not available to those searching with the search engine.

Search engine software is the third part of a search engine. This is the program that sifts through the millions of pages recorded in the index to find matches to a search and rank them in order of what it believes is most relevant. You can learn more about how search engine software ranks web pages on the aptly-named How Search Engines Rank Web Pages page.

Major Search Engines: The Same, But Different

All crawler-based search engines have the basic parts described above, but there are differences in how these parts are tuned. That is why the same search on different search engines often produces different results. Some of the significant differences between the major crawler-based search engines are summarized on the Search Engine Features Page. Information on this page has been drawn from the help pages of each search engine, along with knowledge gained from articles, reviews, books, independent research, tips from others and additional information received directly from the various search engines.