nternal links are the constituent elements of the way your web site is organized. They are the bridges by which one page of your site connects to the other & more importantly outlines, which page of your site will connect to which other page/s.
External links on the other hand are the community tools by which the online community votes for you by linking to you. The more popular you get is a function of more incoming links into your website. This in turn is interpreted by search engines as a measure of your online value creation.
In essence, if your pages are orphaned (no one links to them) or lying isolated within your web site thats not good news from a pagerank point of view. To enjoy a good pagerank for the pages it is imperative to have proper linking amongst various pages within the website as well as having good quality external links coming into the page.
A factor to be cognizant of while linking is that one should focus on quality links & not always quantity. Consider that, there is a huge difference from a link, to your page from another page with a good pagerank than a page from a low pagerank