Directory submissions and search engine submissions differ from each other. Directory submissions require you to submit titles and descriptions about your site under relevant category heads in online directories. On the other hand, submitting your site to search engines would entail submitting your URL for the search engines to locate.

However, it is not necessary to submit your site to search engines, as they will in any case find your site through the links you have on other sites. Directory submissions could help in getting you such links.