A search engine directory is a list of websites that are categorized based on the topics they have, the industries they serve, or the interests they are built on. They are organized to help the search engines discover websites when the user looks for a query.
With proper SEO strategies, the websites can improve their visibility for the search engines to track them and categorize them. This search engine directory keeps updating itself regularly, and there are new algorithms that are continuously being tried to improve the identification of the websites.