Search Engine Optimization is a technique, by which one enables search engines to find and rank your site better than hundreds of thousands of websites in the world wide web, in response to a search query, called Keyword.
Whenever one enters a search query in a search engine and clicks search or hits enter, a list of sites appear. It is general tendency that the viewers visit the sites on the top of the list, as it is perceived that they are more relevant to the search query. Who decides, which sites go on the top of the rankings? Each search engine scans or "crawls" the web reading every page and recording every activity. Sites which are better optimized and relate to a keyword, are favored more than sites which are not. These crawlers visit the site continuously over staggered intervals of time, depending on the level of activity, looking for changes.
The rankings are dynamic. A site can be on the top of the ranking today, and may drop the next time the search engine updates its ranking, for various reasons like drop in activity, competitors promoting their site very actively and the like. SEO, is not a one time solution, it is a continuous process.
SEO is one of the most powerful online marketing tools, which can boost traffic and increase business through your site. However it should be understood that only SEO does not guarantee top rankings. Different Search engines like Google, Bing, Yahoo, Ask, etc., have their own algorithm for finding and ranking sites. A sit owner should be cautious when he/she is trying to optimize his/her site for different search engines.