How search engines work

What is search engine.

Google's search engine is a powerful tool  that gathers and shorts out contents material from all over the internet. Without search engines like Google, it would practically be impossible to find the information you need.Those wishing to find something would enter a query about what they'd like to find. Your search engine will provide links to the content that matches what they want.
Google is known as the most popular search engine among the surfers.There are many other search engines that are also used.they include:
yahoo.com
Dogpile.com
Ask.com
Bing.com

How search engine works.
It means when you enter a query or do any operation on a search engine. How and what it responses in respect to your query?
if you analyze you see that search engine give results on the bases of your query.search engine have two major functions crawling and building an index

Crawling
Crawling is the place where everything starts.the acquisition of data about a website.this include scanning sites and collecting details about each page,titles,images,keywords,other linked pages.  Different crawlers may also look for different details, like page layouts, where advertisements are placed, whether links are crammed in, etc.
But how is a website crawled? An automated bot (called a “spider”) visits page after page as quickly as possible, using page links to find where to go next. Even in the earliest days, Google’s spiders could read several hundred pages per second. Nowadays, it’s in the thousands.
When a web crawler visits a page, it collects every link on the page and adds them to its list of next pages to visit. It goes to the next page in its list, collects the links on that page, and repeats. Web crawlers also revisit past pages once in a while to see if any changes happened.

Indexing
An index is another name for the database used by a search engine. Indexes contain the information on all the websites that Google (or any other search engine) was able to find. If a website is not in a search engine's index, users will not be able to find it.
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