Search engines are website with the ability to perform a research in their database to find websites matching your queries.
Statistics have shown it, it is most used way to find websites.
Most of these search engines are employing reviewers to rate websites and decides which ones will be added. You can not do much about your ranking in these search engines, since human will review your website. To have a good ranking, you need a good site. This is why the usually give better results than spiders. Yahoo is a good examples of what directories are.