What is a Directory ?
A Website directory is a large collection of websites indexed according to a fixed method. Generally, through alphabetically or by the type of categories.
Unlike the Search engines which also store web pages in their data bases, Directory publishers have human editors that personally look at a website and then add it to the list. Whereas search engines uses crawlers and robots to index the websites.

TIP : If you can index your site in a directory, then your website is 100% assured to be listed in all the search engines.