Google, Yahoo, MSN and other search engines need help to discover what your site is actually saying.  Search engines do not employ an army of people to read and categorize each website they find.  To determine how your website might be a great answer in a search engine results page (SERP) they send out small programs to research (or crawl) through your website.  These programs (called robots or spiders) read every word of each page, follow all of your links and then report back to the search engine on the words they find.  Search engines then have a copy or "index" of your website stored away for quick reference when a search request is made.  If all goes well, there is a great match between what a users asks, and what the search engine finds on its copy of your website. Image or Flash driven websites are almost impossible to "read" by a search engine and therefore rarely show up as a good result when a search engine query is made.  Search engine optimization is a balance between website content, link building, and keyword research.