Directories help locate websites within specific or definite categories. The difference between search engines and directories is that, directories feature only vetted websites, sites that have a certain standard and contain information of substance.
One of the aims of using directories is to boost traffic. To achieve this, one must choose to be featured in large, organized directories like Yahoo, DMOZ, and LookSmart. These will drive highly targeted traffic to you site (subject specific), increase significantly the link popularity, and Google PageRank.
To maximize exposure: