Search Engine Optimization (SEO)
A search engine is a large online database that can be searched by entering words or phrases into a search box.
Search engines send programs called spyders out on the internet to search for web sites and index the site pages and follow each link on your web site to gather information about your web site layout and its content and send it back to the search engine's database.
Search engines love fresh, relevant content. Once they discover your site they will periodically check back to see if there is any new content. The goal of any search engine is to provide web surfers with the kind of information they want. To achieve this goal, they use complex algorithms to determine the popularity and page rank of your site.
There are many factors involved but keyword use and density are important factors. Keywords are simply the search terms web surfers use.