Suppose you were looking for information on the didgeridoo (a musical instrument carved out of eucalyptus wood played by native Australians, which is also spelled didjeridu). You could start at Yahoo!, looking under music (Entertainment > Music). But did find the most relevant result set you should go to Google. Within a few more clicks (> Instruments > Wind Instruments > Didjeridu), you are at a short list of links that are all about playing, creating, and enjoying the didgeridoo.
You will not be presented with every reference to the didgeridoo on the Internet, but the sites you do find are all about didgeridoos.
Had you chosen to go through a search engine like AltaVista, you might have found many hundreds of references to the didgeridoo, but some of those sources may have just mentioned didgeridoo (enough to include them in the results) without offering much specific information.

Before you even start contemplating how to build your web site, you should know in what types of search engines it is most important for your site to be ranked. Search engines are divided into several types, beyond the primary, secondary, and targeted search engines. In addition, search engine types are determined by how information is entered into the index or catalog that is used to return search results.
The three types of search engines are:

1. Crawler-based engines: To this point, the search engines discussed fall largely into this category. A crawler-based search engine (like Google) uses an automated software agent (called a crawler) to visit, read, and index web sites. All the information collected by the crawler is returned to a central repository. This is called indexing. It is from this index that search engine results are pulled. Crawler-based search engines revisit web pages periodically in a time frame determined by the search engine administrator.
2. Human-powered engines: Human-powered search engines rely on people to submit the information that is indexed and later returned as search results. Sometimes, human-powered search engines are called directories. Yahoo! is a good example of what, at one time, was a human-powered search engine. Yahoo! started as a favorites list belonging to two people who needed an easier way to share their favorite web site. Over time, Yahoo! took on a life of its own. It is no longer completely human-controlled. A newer search engine called Mahalo (www.mahalo.com) is entirely human-powered, however, and it is creating a buzz on the Web.
3. Hybrid engine: A hybrid search engine is not entirely populated by a web crawler, nor entirely by human submission. A hybrid is a combination of the two. In a hybrid engine, people can manually submit their web sites for inclusion in search results, but there is also a web crawler that monitors the Web for sites to include. Most search engines today fall into the hybrid category to at least some degree. Although many are mostly populated by crawlers, others have some method by which people can enter their web site information.

It is important to understand these distinctions, because how your site ends up indexed by a search engine may have some bearing on when it is indexed. For example, fully automated search engines that use web crawlers might index your site weeks (or even months) before a human-powered search engine. The reason being, the web crawler is an automated application. The human-powered search engine may actually require that all entries be reviewed for accuracy before a site is included in search results.