Search engines 'are mechanisms which aid users to search the entire Internet .for relevant information. They are based on allowing users to enter keywords that are matched against a database. Unlike subject directories, which use human indexers to build their indexes, search engines use software programs which create automatically their own databases containing lists of web pages. Search engines are composed of three different parts:
i) a program called a spider (or robot or crawler),
ii) a database with an index and
iii) search software.
Spiders wander through the web, crawling from site to site, following links between pages. Different search engines use different types of spiders, whereas some visit every possible site, others are based on more selective principles and visit only the most popular sites. The first type of spider finds a huge volume of information in a short space of time; the second type generates a smaller number of pages with perhaps more relevant information. Every page found by a spider is stored in a database and an index of its contents is built by extracting automatically words from the web pages and ranking them alphabetically using some principles which inverted files use. The index is, therefore, a list of every word (except stop words) with a pointer to its location on the database.
Again, different search engines will follow different principles. For example some will index every single word on every web page, while other search engines index only the title and top level phrases of a web site. The third element of the search engine is the search software, which is a program, which compares search queries keyed in by the people (users) with the index, finds matches and ranks them in order of relevance. The criteria for judging relevance will vary according to the search engine.
The different approaches, which search engines use in crawling the web, finding new pages and indexing them will .produce totally different results. That is why when searching the same topic in different search engines there can be differences in the results obtained. Due to the way the search engines operate search engines are more oriented to find larger volumes of information and more specific information than subject directories. This is because they are based on searching Web pages and indexing automatically precise words from the sites (rather than using a pre-defined index) as subject directories do.
Examples of search engines include:
- Alta Vista (http://altavista. Digital. Conti)
- Excite (http://www. Excite. Corn/)
- Hot Bot (http://www. Hot bot, Corn/)
- Infoseek (http://www. Infoseek. Corn/)
- Lycos (http:www. Lyeos. Conti)
As different search engines possess different strengths, new search tools have been devised recently, which enable people to search simultaneously the different databases of search engines, while using one single interface. These tools are called multi-threaded search engines. Though, these do not have all the search flexibility of individual search engines, they are very fast and can search through vast quantities of information. Some examples of multi-thread (meta) search engines are:
i) Dogpile (http://www. Dogpile.com)
ii) Metacrawler (http://www. Metacrawler com)