A very big part of SEO is working on links. You need to create all kinds of links in order to get better SERP (Search Engine Results Page) rankings. However, for all that we talk about needing to gain social rank, i.e. getting links from outside web sites, building links within your own website is at least as important as building them outside your website. These are known as internal links and they are a vital SEO tool.

Two Kinds of Internal Links
Before we continue on to why internal links are so important for search engine optimization, we should take a moment to discuss what they are. There are actually two kinds of internal links and they serve vastly different functions (though both can be extremely useful to your web site):
Within a Document
The first kind of internal links provide links within a single document. This can extremely useful for your visitors if you have long documents, for example a small ebook made available online. These kinds of links, while not particularly important for SEO purposes should nonetheless be part of any site building effort as they can ensure that you will have easy to read content.
Within a Website
The second kind of internal link, and the kind we’re primarily interested in as a company that specializes in SEO is internal links within a website. These links provide a connection from one point in your website to another point in your website (i.e. you might find links at the bottom of an article mentioning related articles.)
People Spend More Time on Websites With Good Internal Links
The first reason that you must have internal links is that by offering links to additional material on your website, you are much more likely to have repeat readers. These people will keep coming back to your website and will look at everything you have to say because you provided links to other articles they may be interested in.
Internal Links Help Google to Crawl Your Entire Website
In addition to getting visitors to spend more time on your website, having internal links also helps the search engines such as Google and Yahoo! to spider your web site properly (they follow links through your website in order to index all the different pages on your site. If there are no links, you will see no spidering).
What This Means for SEO Purposes
The bottom line when it comes to SEO is that the articles and other content buried deeper inside your website may very well become the thing that gets people to click a link from the search engine to look at your site. It is not uncommon for example for websites to find that keywords they didn’t target are suddenly coming up on the front page of SERPs simply because they made sure to include such links.