Search Engine Optimization, commonly referred to as SEO, is the process of optimizing, or configuring a website or webpage for natural search engine indexing and ranking. In other words optimized the webpage or website is, the easier it is for search engines to find it.
Search engines use algorithms (code) to match the word or key phrases, often referred to as "keywords" or "keyphrases", that a user types into the search field with the webpages, files and websites that it has stored knowledge of. These algorithms look for specific data and attempt to return the best match for what it thinks the user is searching for. Given a search engine such as Google, this process happens millions of times a day in seconds.
Some of the specific data the search engines are looking for falls into two primary parts. These parts are known as "on page" seo and "off page" seo.
On page seo is basically configurations that are made on the webpage or website itself. On page seo generally deals with such areas as optimizing the title tag, the anchor tags text, the meta keyword tag, the meta description tag, and the contents in such a way that the search engine can read the source code and content of the webpage or website, and know whether or not it is relevant to what the user is searching for.
Off page seo is basically seo actions that are performed outside the realm of the webpage or website itself. Off page seo is the task of obtaining inbound links from other websites or webpages that are themselves optimized and well ranked for the same keywords or keyphrases. When the search engines detect that there are inbound links to your webpage that have similar topics to your website or webpages theme, they will use this data during the analysis to help determine relevancy to the users search.
Note that there are other things that can be done to optimize a website or webpage that are not mentioned here that are beyond the scope of this introduction. Also, note that the list is every changing in terms of what it includes and level of importance.
Finally, the ultimate goal of the search engines is to provide relevant results to the users original query. Applying the factors listed here while writing naturally flowing but relevant content for websites or webpages is SEO – Serach Engine Optimization.