Search engine optimization (SEO) is the process of improving the volume or quality of traffic to a web site or a web page (such as a blog) from search engines via "natural" or un-paid ("organic" or "algorithmic") search results as opposed to other forms of search engine marketing (SEM) which may deal with paid inclusion.
More Info:
SEO on Wikipedia : http://en.wikipedia.org/wiki/Search_engine_optimization
SEO on Google : http://www.google.com/support/webmaster ... swer=35291