Search engines check out your site and catalog the information found there so that when browsers are searching and match a key word or phrase found on your site a like to your page will come up. That means that the more information you have on your site the more you improve your search engine optimization (SEO).
Keywords: There are programs that will help you ensure that you are utilizing the proper keywords on your site. Wordtracker, for example keeps tracks of the common searches performed by people. Once you have knowledge of the words that people are typing into their search on the subject of your product or service you can incorporate those words in your website as often as possible to optimize the benefit you gain from search engines.
Keyword Density: This is the calculation of frequent use of the keywords on your site. You want to always be sure you are utilizing keywords or phrases as much as possible, in articles and all over your site. While using as many keywords as possible you want to be sure that the information on your page flows well and is interesting.
Variety of Keywords: You in no way want to limit the benefit you gain from search engines. You need to use a variety of keywords or phrases under a variety of subtopics on your page. If you have articles or pages on different subtopics under your one main topic research to see what searches are being performed on those subtopics and utilize those keywords as much as possible through your site as well. This will draw a great deal of traffic to your site.
Packed Full: Readers skim rather than read but you still need to include a lot of information. Search engines are looking for a lot of content and you want to give them what they want. Including a lot of information in a way that the reader will not feel overwhelmed. Break the information up into small paragraphs, or use lists and bullet points to draw their eyes to the main points. Once you draw the reader in and peak their interest they will read on.
Headings: If you are trying to use a lot of keywords and phrases in your page you may find that you are becoming repetitive. Use headings and subheadings in your writing to break things up a bit.
Close Gaps with Keywords: If you are trying to use the same words over and over again your writing could get boring. Write an interesting and exciting article and then go back and insert the keywords or phrases.
The Reader is Equally Important: While you want to cater to the demands of the search engine, remember that the browsers of your site are your consumer. You want to cater to them as well. So give a lot of information but keep it enjoyable for the reader. If they find you through a search engine but do not like what they read when they get to your site you have not benefited yourself at all.