Role Of Search Engines - How Search Engine works?

It's really very interesting concept in front of everyone that how search engine's work? what is their criteria to select best one?
Definitely Search Engines follows some algorithm to crawl any webpage and these web crawlers indexing our webpages after some specific period of time.Sometime the website owner think that he has a perfect website with a good content, but in SERP the ranking of this site is not good, The site owner can't understand why the site is not getting the good ranking in google, Yahoo and all other major search engines.

First we must understand what is webcrawlers?

We can define web crawler is a kind of bot, or Agent/software agent And these web crawler starts with a list of URLs to visit, called the seeds. As the crawler visits these URLs, it recognise all the hyperlinks in the page and adds them to the list of URLs to visit. And our main emphasis is on to create our site's link on to the other website's with a relevant theme.

Some specific points which is really beneficially -:

1)How Webpages to be crawled by Search Engines?:- Search engines send out automated programs, sometimes called “bots” or “spiders,” which use the web’s hyperlink structure to “crawl” its pages and adds them to the list of URLs to visit.

2)Indexing Of Documents By Search Engines:- After the work of crawling a page, Now the pages are stored in a database in a very good manner that makes up the search engine’s index. These indexes contain billions of documents, which are delivered to users in mere fractions of a second.
When user put on some keyword or any other thing for which they want to see result than Search Engines gives them best to best result regarding their query from the collected billion's of data.

3)Query processing. When a user want to search something through search engines, then the Search engine examines its index to find documents that best match. Every time when a user can change its query it will show the different result according to the query of the user.Let be understand by an example:-If a user searching the phrase “Software Companies,” without quotes around it, yields more than around Twenty million results in Google. Do the same search with the quote marks, and Google returns only 75, 000 results.

4)Results Of Ranking:- Search engine runs an algorithm on the results to calculate which ones are most relevant to the user's query. That are shown first, with all the others in descending order of relevance.