Here I am going to define the Search engine with respect of two view as given below :
1. In the respect of Visitor (Who want getting best results)
2. In the respect of Website (Who want make best results)
In the respect of visitor: the Search engines enable to their visitors to enter a specific word or term, known as keywords. Once submitted, all pages containing those keywords that can be found in the search engine’s directory are listed on the search engine result pages.
In the respect of website: who wants listing to search engine a search engine is a website such as Google on which people can search for other websites on the Internet. Search engines do not include sites manually, but have an automated “search engine spiders” that “crawls” the web through links. If your website has just one link to it, it will eventually be found and indexed in a search engine. The term “search engine” is often used generically to describe both crawler-based search engines and human-powered directories.
There are two type of Search Engine:
Crawler-based search engines
These two types of search engines gather their listings in radically different ways.
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.
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.