Crawler-Based search engines randomly index pages on the web. They are not exhaustive and different engines will index different sites.
To use a search engine you must understand how your results are arranged. Many search engines will arrange results according to the number of times your search terms appear in the document. The more a term appears on a web page, the closer that page moves to the beginning of your search results. Some sites will give priority if your terms appear in the title. With some, not all your terms have to appear in the document. That rare medical term might not be present in your first few results. Examples of some popular search engines are Google (http://www.google.com), AltaVista (http://www.altavista.com), and AllTheWeb (http://www.alltheweb.com).
Google is the top choice for web searchers. It indexes the largest collection of web pages of any crawler-based search engine and it uses a unique method for ordering the results, ranking them according to the number of other sites that link to the target document - a type of peer review.
Google provides a search button you can place on your Mac or PC Web browser toolbar. This allows you to initiate a search without having to first go to the search engine's web site. This is a real time saver.