It is a common misconception that, when you submit your query, you are actually searching the Internet and this is not true. You are searching that search service's database of sites and documents that it has collected from the Web and other sources.
To increase the amount of data that you can search through, many Web directories offer a search engine's database in addition to their own, and search engines have begun to display directory-like categories on their main page. Many search engines have partnered with the Open Directory Project for their hand-selected data.
There are typically two components that make one search service different from another, the software that searches in response to your query and the database of indexed Web sites and other data. The same query submitted to different searching services can return results that are very similar, somewhat similar, or very different, all depending on the data the services have in their databases.

The basic concept of a search engine:
Type a word or phrase into a search box and click a submit button. Wait a second, and references to thousands (or hundreds of thousands) of pages will appear. Then all you have to do is click through those pages to find what you want. But what exactly is a search engine, beyond this general concept of search?
On the back end (in the database tier), a search engine is a piece of software that uses applications to collect information about web pages. The information collected is usually key words or 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.

Every search engine contains or is connected to a system of databases, where data about each URL on the Web (collected by crawlers, spiders, or robots) is stored. These databases are massive storage areas that contain multiple data points about each URL. The data might be arranged in any number of different ways, and will be ranked according to a method of ranking and retrieval that is usually proprietary to the company that owns the search engine.

Click the Exercise link below to observe how different search services respond to a search that yields no results.
Searching Databases - Exercise