Internal linking, links within a website, if done properly can greatly contribute to the success of your website in the search results.
There are to areas to consider when thinking about your internal linking strategy; navigational links and in content links.
Another thing to think about is the logical structure of your link and the keywords used in them. Always ensure that your strategic keywords are always included in your navigational links.
In content links are also very crucial to search engine rankings and contribute very positively towards your overall internal linking strategy, if implemented properly that is.
The think to remember when thinking in-content links is Wikipedia. The reason Wikipedia is so successful (besides the thousands of unique content pages) is the way it links these pages together, using text links pointing back to the dedicated page of the word being mentioned.
And thats about it really, be smart about how you link your pages, not only for the search engines but also for your users.