SEO or Search Engine optimization is a procedure or a principle in helping a certain website increases its traffic, improving its page rank and giving your website more chances of getting the attention of the researchers.
Backlinks are links that are directed towards your website. Also knows as Inbound links (IBL's). The number of backlinks is an indication of the popularity or importance of that website. There are two types of backlinks which is internal and external backlinks.
External links tell the search engines what pages on your site are considered as important by other people. Links pointing to your internal pages are known as deep links. The more deep links you get the more traffic you will attract to these internal pages. When obtaining deep links make sure the anchor text is related to the content of the internal page.
Internal links are not weighed anywhere near as heavily as external links, but they do count towards pagerank. Internal links are important too; they tell the search engines which pages are important to you on your site. You can see the effect especially when you have hundreds or thousands of your pages indexed. So the internal linking is also important for gaining overall high pagerank. In your website design, you should try and have a text link back to your homepage from every page of your site. For internal page factors, pay more attention to the title tag, the meta tag, and the URLs.