Search Engines are basically tools that are used to search for information on Web. Okay, so you know the basic concept of a search engine. Type a word or phrase into a search box and click a button. After a Wait of few seconds, thousands of references to thousands (or hundreds of thousands) of pages will appear. Only thing you have to do is to click through those pages to find what you want.
On the back end, a search engine is a piece of software that uses applications to collect detailed information about web pages/sites. This information collected is usually consists of key words or key phrases that are possible indicators of what is contained on the web page as a whole, the URL of the page, the code that makes up the page, and links into and out of the page. That information is then indexed and stored in a database.
On the front end, the software has a user interface where users enter a search term — a word or phrase in an attempt to find specific information. When the user clicks a search button, an algorithm then examines the information stored in the back-end database and retrieves links to web pages that appear to match the search term the user entered.