Article directories are websites that house free articles. These articles are usually put there by whoever owns the copyright to them as a promotion method. Each article has the owners byline placed under it so that those who eventually read the article will know who wrote it (or at least who owns it) and will be able to get in contact with or visit their website for further information.
Articles directories are a great place to put your articles so that they will make you money. By using this method, people who are interested in your product or service will see your article, view you as an expert in the subject, and will visit your website. Even if you cannot write or do not have any knowledge on the subject on which you are promoting (for example for you run an affiliate website), you can always hire a writer to do the work for you. Getting quality articles written will be expensive up front, but they will pay for themselves in no time, either through affiliate sales or selling your own services, and everything from then on is pure profit.