Directory submissions are useful to announce your website to the web and it does build backlinks but most directory submissions are of very little power as far as backlink power except in the paid inclusion directories such as DMOZ (Free but can take months to get listed) and Yahoo (VERY EXPENSIVE).
Backlink building is a whole other subject....to really increase your pagerank, you need to get your page linked onto other sites that are somewhat related, have pagerank and are do follow. This can be very time consuming. I think if you will visit SEO IT! 's hubs I think you will find more detailed information.