Part two in our series on Search Engine Optimization (SEO) covers the basics of internal linking. If you remember the pages that you created in part one (corresponding to the specific keywords and phrases you are targeting), the next step is to create links from all the places you reference a keyword or phrase to the appropriate landing page.
This can include keywords that appear in the body of other content but you may also want to consider adding menu items which contain keywords or phrases and are linked to those pages. If you have the ability to set up a "tagging system" to allow you to tag content with keywords and keyphrases you can help associate your site with a wider range of keywords in addition to the main keyphrases you are focusing on. This can also help your end users because tagging related content will allow you to pull other relevant content in to pages that share tags.
Essentially what internal linking does is explain to Google and other search engines which pages you consider to be the authoritative pages for the keywords that you link from. Google keeps track of how many incoming links the page has, and which keywords it is linked on. In general, the more links the better (especially if they are linked with the relevant keywords). Google loves it when you can tag related items with keywords and gobbles up the landing pages that you get when you click on a keyword.