Here is a interesting video on “How Search Works?” by Matt Cutts
SEO stands for “Search Engine Optimization.”
All major search engines alike Google, Yahoo and Bing have primary search results.
On the other hand, web pages and other content such as videos or local listings are shown and ranked based on what the search engine considers most relevant to users.
The best part of SEO is that payment is not involved, as it's with paid search ads.