How Search Engines Work
Search engines use programs called spiders, crawlers, or robots that follow the links from page to page and download a copy of each one found.
Pages are then analyzed, indexed, and added to it's database. Websites are crawled at different rates. the more they like the site the more often they come back. Use of interesting content and keywords are a large factor. You can also set the time which you would like to be crawled.Information which the search engine operators view as important to pass on is added to the analysis of the results of specific keyword searches.
An analysis of an existing web site can greatly improve the site’s ranking for specific searches. Then recommended changes are made to increase ranking.
Changes such as; content of the pages, image tagging, keyword and keyword phrases, title tag, URL structure, editing or limiting the number of characters in any of the a fore mentioned will optimize your website.
KEYWORD DENSITY AND HOW IT APPLIES:
Count is the number or words in the content. They then take the keywords and divide to see the ratio of keyword to content count.
Although there is no exact "correct number" it seems the best density is between 7% to 15% where 7 shows there is importance to the word as related to your content and 15 is not yet spamming or word stuffing. Once you reach those numbers we compare your position on the google pages to your competitor and may have to increase the density to move you up.
Think of it as making home made soup or sauce you add a pinch here and there as needed until you get it just right.