When you query a search engine to locate information, you're actually searching through the index that the search engine has created – you are not actually searching the Web. These indices are giant databases of information collected and stored and subsequently searched. Different search engines also use different criteria for data collection. So an exact search on one engine will yield different results than on another search engine. Different algorithms will give you different results. The algorithm is what the search engines use to determine the relevance of the information in the index to what the user is searching for. A simple example of a search engine algorithm would be Content Keyword Density.

One of the elements that a search engine algorithm scans for is the frequency and location of keywords on a Web page. Those with higher frequency are typically considered more relevant. But search engine technology is becoming sophisticated in its attempt to discourage what is known as keyword stuffing, or spamming. This is one of many "black hat" techniques or "unethical search engine optimization practices."