What are Search Engines and How They Works ?

Definition:-
A search engine is a searchable online database of internet resources. It has several components: search engine software, spider software, an index (database), and a relevancy algorithm (rules for ranking). The search engine software consists of a server or a collection of servers dedicated to indexing Internet Web pages, storing the results and returning lists of pages to match user queries. The spidering software constantly crawls the Web collecting Web page data for the index.
Some Search Engines: AltaVista, Google, HotBot, Lycos, MSN, Yahoo, AOL.

Types of Search Engine:-

1.Crawler-Based Search Engines:- Crawler-based search engines, such as Google, create their listings automatically. They "crawl" or "spider" the web, then people search through what they have found. If you change your web pages, crawler-based search engines eventually find these changes, and that can affect how you are listed. Page titles, body copy and other elements all play a role.

2.Human-Powered Directories:- A human-powered directory, such as the Open Directory, depends on humans for its listings. You submit a short description to the directory for your entire site, or editors write one for sites they review. A search looks for matches only in the descriptions submitted. Changing your web pages has no effect on your listing. Things 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.

3.Hybrid Search Engines:-In the web's early days, it used to be that a search engine either presented crawler-based results or human-powered listings. Today, it extremely common for both types of results to be presented. Usually, a hybrid search engine will favor one type of listings over another. For example, MSN Search is more likely to present human-powered listings from Look Smart. However, it does also present crawler-based results (as provided by Inktomi), especially for more obscure queries.
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Parts of a Crawler-Based Search Engine:-

Crawler-based search engines have three major elements.
1. Spider:- It is also called the crawler. The spider visits a web page, reads it, and then follows links to other pages within the site. This is what it means when someone refers to a site being "spidered" or "crawled." The spider returns to the site on a regular basis, such as every month or two, to look for changes.
2. Index:- Everything the spider finds goes into the index. It is sometimes called the catalog, is like a giant book containing a copy of every web page that the spider finds. If a web page changes, then this book is updated with new information.
3. Search engine software:- This is the program that sifts through the millions of pages recorded in the index to find matches to a search and rank them in order of what it believes is most relevant.

How Search Engines Rank Web Pages:-

Search for anything using your favorite crawler-based search engine. Nearly instantly, the search engine will sort through the millions of pages it knows about and present you with ones that match your topic. The matches will even be ranked, so that the most relevant ones come first.

Of course, the search engines don't always get it right. Non-relevant pages make it through, and sometimes it may take a little more digging to find what you are looking for. But, by and large, search engines do an amazing job. Unfortunately, search engines don't have the ability to ask a few questions to focus your search, as a librarian can. They also can't rely on judgment and past experience to rank web pages, in the way humans can. So, how do crawler-based search engines go about determining relevancy, when confronted with hundreds of millions of web pages to sort through? They follow a set of rules, known as an algorithm. Exactly how a particular search engine's algorithm works is a closely-kept trade secret. However, all major search engines follow the general rules below.

Location, Location, Location...and Frequency
One of the the main rules in a ranking algorithm involves the location and frequency of keywords on a web page. Call it the location/frequency method, for short.
Pages with the search terms appearing in the HTML title tag are often assumed to be more relevant than others to the topic.Search engines will also check to see if the search keywords appear near the top of a web page, such as in the headline or in the first few paragraphs of text. They assume that any page relevant to the topic will mention those words right from the beginning.

Frequency is the other major factor in how search engines determine relevancy. A search engine will analyze how often keywords appear in relation to other words in a web page. Those with a higher frequency are often deemed more relevant than other web pages.

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