Everything you need to answer your questions
SEO (search engine optimizer)SEO is a set of strategies with the ultimate goal of increasing the positioning of a website on natural (organic) results pages on search engines like google.com.
It is this procedure that makes your website appear more positioned on the main search engines.
The better the SEO of your website, the more ranking this will have on the search engines.
The goal of a good SEO is to make what your website appears in the first few pages of the search engines.
Do not confuse natural (organic) results with paid advertising. (ex: Google AdWords)
For more information on SEO see the Wikipedia website.