Links as votes is a helpful analogy to remember. Basically, if Page A links to Page B, then Page A is casting a vote that Page B is relevant to the keywords used in the link text or anchor text connecting the 2 pages (example: in the link to Name.com at the end of this sentence the "link text" or "anchor text" is domain name). In addition, votes can be stronger or weaker depending on factors like overall trust of the link, PageRank of the page it came from, and even how significant the relationship between Page A and Page B is.
In other words, the more votes that Page B gets, the higher the likelihood it will rank higher for keywords. The more internal links you provide that point to any given page on your site, the more opportunities there are for that page to rank highly for its targeted keywords. Here are three important internal "link structures" and some simple tactics to optimize them.
1. Global Navigation - The template for global navigation (including home page alt tag) is one of the most elemental starting points for SEO. This is especially true for large sites because the more pages your site has, the more votes the global navigation template is providing for you. This is not to say that you need to have lots of pages. By putting the right keywords in global navigation links like the drop-down menu, your site is more likely to get credit for any internal links. That's why indexing the links is both important and helpful.
2. Link subsets - There can be any set of links existing on the page template which specifically targets page groups with targeted keyword phrases. This is an ideal vehicle for creating that ever-important link connectivity to the high priority pages. Typically, this is in the footer area or on the right column of the page. Sites may have buckets for "Related Topics" or "Most Popular Pages" or a similar subset which is both related to the main link and relevant. Look at the Name.com Footer for an example of how this is done.
3. Navigate the breadcrumbs - Breadcrumb navigation is an internal link structure that can enhance SEO; especially for any sub-pages that are not linked by global navigation. Any links appearing in the global navigation template will typically occur in the source code before any breadcrumb navigation or content links. This means that global navigation links supersede any other links on the page. Mainly because most search engines (like Google) only count the first link found in the source code between Page A and Page B. Any breadcrumb navigation links must be keyword focused as the primary purpose of them, SEO-wise, is to link to any pages outside of the global navigation template.
Remember that the main purpose for all three of these types of link structures is to direct users to the right information. When the link structures are keyword focused, it aids search engines and users in finding the relevant information.