Search Engines like Google, Yahoo! and Bing are being used by people in order to find things or information online. When a person type in a phrase on the search box, the search engine will then come up with a result related to the phrase or keyword. In most cases, these search engines do give away two typs of listings. One is the free or natural listing and the other is the paid listings such as PPC.
What a search engine does is that it ranks the results in as per relevance to the keyword the user typed on the search box. The most relevant does appear on the number one spot. Every search engine has its own algorithm in computing the result both for free and paid. As to what search engine the user go for, this would be dependent on what his preference.