Although the term "search engine" is often used indiscriminately to describe crawler-based search engines, human-powered directories, and everything in between, they are not all the same. Each type of "search engine" gathers and ranks listings in radically different ways.

Crawler-based search engines such as Google, compile their listings automatically. They "crawl" or "spider" the web, and people search through their listings. These listings are what make up the search engine's index or catalog. You can think of the index as a massive electronic filing cabinet containing a copy of every web page the spider finds. Because spiders scour the web on a regular basis, any changes you make to a web site may affect your search engine ranking.

It is also important to remember that it may take a while for a spidered page to be added to the index. Until that happens, it is not available to those searching with the search engine.

Directories such as Open Directory depend on human editors to compile their listings. Webmasters submit an address, title, and a brief description of their site, and then editors review the submission. Unless you sign up for a paid inclusion program, it may take months for your web site to be reviewed. Even then, there's no guarantee that your web site will be accepted.

After a web site makes it into a directory however, it is generally very difficult to change its search engine ranking. So before you submit to a directory, spend some time working on your titles and descriptions. Moreover, make sure your pages have solid well-written content.

Some search engines offer both crawler-based results and human compiled listings. These hybrid search engines will typically favor one type of listing over the other however. Yahoo for example, usually displays human-powered listings. However, since it draws secondary results from Google, it may also display crawler-based results for more obscure queries.

Most search engines offer some form of paid inclusion. Paid inclusion guarantees your web site gets reviewed and/or indexed promptly. Keep in mind that these search engines still allow people to submit for free; it just takes longer. Pay per click search engines are different. Their listings are comprised entirely of advertisers who have paid to be there. With services such as Yahoo SM, Google AdWords, and FindWhat, bids determine search engine ranking. To get top ranking, an advertiser just has to outbid the competition.