Search Engine Optimization
Search engine optimization (SEO) is the process of improving (and maintaining) the "organic" search result position of a website. Usually, the better a website is positioned, the more likely people will visit the site. And the best place to be for any search phrase is at the top of the first page.
Now, SEO can also target different kinds of searches, including an image search and map (local) search.
Search engine optimization is a very complicated topic. There are hundreds of factors that lead to good positioning. It's not as simple as good keywords and meta tags anymore; now we consider things like the number of instances a keyword appears on a single page in relation to non-keywords, whether or not it's bold, in an heading tag, or is a link. And search engines, like Google, change their "rules" (algorithms) all the time. So it's very difficult to keep up on exactly what they look for.
However, good, legitimate, and easy-to-find content is what they want; not SEO tricks.
Google's SEO page