A Search Engine is a computer program that searches a resource to return a response for specific inquiries. An attempt to bring organization to chaos. An Index used to simplify locating relevant information and presenting it in a useable or familiar format, giving those results in an organized fashion listing the most relevant results first to the most obscurely remotely relevant results last.

The program is initiated when a query is put to the program in the form of "find information related to ____". The blank would be a Key word, or the variable.

In your local library the Search Engine could be the card catalogue or the library computer.

The Dewey Decimal System was developed in the late 1800's to assist organization of a resource books, into a system of uniform storage and retrieval. This system of organized recording, storage and retrieval is a form of search engine.
With the advent of computers now days many libraries can link together to enlarge their available resources without having to duplicate their inventory to match every other library.

On the internet, the resources are not a finite number of books in a physical location but a constantly growing network of information in the form of individual websites. The library catalogue does not normally contain a complete copy of every book in its index, however Search Engines do. The more sophisticated Search Engines actually contain several versions of a particular website, captured over a period of time and stored in a way to track the history of that website.

A Search Engines value is determined by its end user, you, and your determination of the relevance of the response it generates to a query. If you found the results to your search less than helpful, you would go to another Search Engine.

In other words, Search Engines are designed to provide relevant answers to search queries.
Search queries are framed by looking for related terms or "Keywords."
How a modern Search Engine does this is by sending out a program of its own to search the internet by following connections from one website to another (links). This search engine initiated search program is often referred to as a Spider.

The Search Spider is not a human. So it does not read or follow the information as a human would. It must read not only the content of the page that a human can see, but also the code that holds the internet together. The Hyper Text Markup Language, or HTML.
HTML frames the website in a uniform way so that your browser knows how to present it to you the human user.

By following links from one website to another, the search spider finds new information, and determines its relevance. Simply stated the Search Spider scans the text of a given website, both the visible and coded, makes a determination of its contents and gives the page a numerical value. That value is then added to or taken away from based on the number of other websites linking to that page and the quality of those pages.

Because the people behind the Search Engines know that they must present the best most relevant information for any given search, they work hard to adjust the program to be as accurate as possible at all times. Because there is so much money at stake, through revenue generated by business online, there are individuals that would attempt to falsely manipulate those search results to favor their websites. For this reason, the Search Engine Program is zealously guarded by its owners and continuously adjusted to avoid being fooled into presenting an irrelevant result.

This is why business owners need to have Search Engine Optimizers A SEO Pro to present their website in a way that the Search Engines can find it relevant and trustworthy enough to rank well in Search Results.