Two type of directories.
(1)Free..In free directories you can submit your website for no cost or any exchange of links. The main disadvantage of free directories is that it will take a lot of time to increase your PR (page ranking) and get heavy trafic! There are over 3500 (plus) free directories that you can find on the Internet and increasing weekly.
(2)Paid directories...Paid directories are manually administered (also some free directories). An editor will review your website/blog before entering into their directory. So if you have the budget, paid directories are a good place to start promoting your website/blog. There are several paid directories on the Internet and can be found from doing a search! Before submitting to paid directories, check the PR and Traffic of the directory. You can check pagerank of any website by using a free tool available at checkpagerank.com. To check traffic use alexa.com. The PR of the page, is rated on a scale of 1-10. Alexa displays the rank and traffic of the website according to their stats.
In Search engine optimization (SEO) and Internet marketing "directory submission" are important, if used properly!