Three Types of Search Engines
The term "search engine" is often used generically to describe crawler-based search engines, human-powered directories, and hybrid search engines. These types of search engines gather their listings in different ways, through crawler-based searches, human-powered directories, and hybrid searches.
Crawler-based search engines
Crawler-based search engines, such as Google (http://www.google.com), create their listings automatically. They "crawl" or "spider" the web, then people search through what they have found. If web pages are changed, crawler-based search engines eventually find these changes, and that can affect how those pages are listed. Page titles, body copy and other elements all play a role.
The life span of a typical web query normally lasts less than half a second, yet involves a number of different steps that must be completed before results can be delivered to a person seeking information. The following graphic (Figure 1) illustrates this life span (from http://www.google.com/corporate/tech.html):
1. The web server sends the query to the index servers. The content inside the index servers is similar to the index in the back of a book - it tells which pages contain the words that match the query.
2. The query travels to the doc servers, which actually retrieve the stored documents. Snippets are generated to describe each search result.
3. The search results are returned to the user in a fraction of a second.
A human-powered directory, such as the Open Directory Project (http://www.dmoz.org/about.html) depends on humans for its listings. (Yahoo!, which used to be a directory, now gets its information from the use of crawlers.) A directory gets its information from submissions, which include a short description to the directory for the entire site, or from editors who write one for sites they review. A search looks for matches only in the descriptions submitted. Changing web pages, therefore, has no effect on how they are listed. Techniques 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
Today, it is extremely common for crawler-type and human-powered results to be combined when conducting a search. Usually, a hybrid search engine will favor one type of listings over another. For example, MSN Search (http://www.imagine-msn.com/search/tour/moreprecise.aspx) is more likely to present human-powered listings from LookSmart (http://search.looksmart.com/). However, it also presents crawler-based results, especially for more obscure queries.