Back Links 

Back links, both in volume and quality, are vital to getting your site on the first page of the search engines.  Below are a number of articles explaining the various factors related to back links.

Anchor text 

Anchor text is the word or words used in the link pointing to your site.  By having relevant anchor text in the links pointing to your site, you will rank higher in the search engines for those keywords.  Not so long ago, you could rely solely on anchor text to rank a site. 

Deep Links 

Deep links are regular back links, but they point to ‘internal’ pages of your site.  This means, they point to a page other than the home page, for example www.yourdomain.co.uk/products.html.  The benefit of deep links is that it raises the search engine rank of that particular page, and in turn gives you more traffic.  Most links will point to your home page, and that’s fine, but you are limited to the few keywords of that single page.  With a good number of links to internal pages, you have many more keywords available to rank for, hence a massive increase in traffic. 

Deep links also increase your site wide Page Rank, again pushing up ranks and ensuring regular visits from the search engine spiders.  A further benefit is getting your entire site indexed, or found, by the search engines.  This creates still more opportunities for good rankings and the traffic that come with it.

The definition...
"Deep linking is the act of placing a hyperlink on a Web page that points to a specific page or image within another website, as opposed to linking direct to the main or home page. If you were to link to this page here, that would be considered a deep link."