Hello, SEO means (Search engine optimization),it is a process of affecting the visibility of website in a search engine's natural or unpaid ("organic") search results.
SEO is a branch of Internet Marketing. It is free inclusion and get a site on high rank in search engine organic results, drive targeted visitors and increase site visibility & popularity.
Search Engine Optimization is the process of increasing the traffic to our websites by getting the higher rank placement in the search engines page!!
The following website will give you a very good answer on SEO enjoy http://www.seomoz.org/beginners-guide-to-seo
That's one big question that can only be answered by reading books and visiting websites. In a nutshell, it's a way to increase your online visibility.
This is a good philosophical question... What SEO really is. We all know that is Search Engine Optimization, but does anyone know all of it?