Search Engines, Google, Yahoo, Msn, Ask are designed to search for information through the web. There are two types of search engines: search engines and directories. The difference is on how result are searched. We can separate them in four main types, crawler-based search engines, directories, hybrid engines and paid search engines
Crawler-Based Search Engines: spiders or web crawlers, they operate through software in order to automatically identify results to our search. An example of a crawler based search engine is Google.
You should remember is that a spider cannot ready flash, so even if you create impressive graphics using flash for your customers or JavaScript spiders cannot identify the content and it will skip this part of your page. As a result your page will rank low and it will not be visible. When you write your contents you should remember that even your human visitor will be attracted from a page full of contents, a spider values the text closest to the beginning of your page and the end. On the other hand spiders can read your meta tags (meta tag description, meta tag keywords etc).
When a spider will crawl your page it will leave a record on your server. The next stage is to compress the results and store the web pages in a search engine index. All the data are stored in the search engine index in a way that makes quickly to identify if a page is relevant to a particular search and to include in the search results with the most common words and phrases on the indexed page. The search results are according to how relevant your page is to these terms, according to a number of internal and external factors.
Spider-Based Search Engines: they use directories as a source to identify new pages to crawl.
Human-Edited Directories: Human-edited directories require manual submission. After you submit your website it will be reviewed from an editor and it will prevent pages from being submitted automatically. An editor will evaluate your website according to the directory’s guidelines and the editor will decide if he/she will include your website in the list. Dmoz is one of the most important human-edited directories.
Hybrid Search Engines, they evaluate sites and scan them to identify for matches. Even Google and Yahoo a directory and crawler search engines, are also hybrid engine. A hybrid search engine like Yahoo usually will present human-edited search and Google will present crawled listings.
Meta Search Engines: MetaCrawler and DogPile are meta search engines. They will present a combination of results from search engines like Google, Yahoo, Msn and Ask.
Paid Search Engines: There are few paid search engines. There a few advertisement programs that offer pay-per-click and pay-per-performance search results. Google with Google Adwords, Yahoo and MSN they have programs for Paid Search Results.