Sites which collecting and gather articles from many subjects then sorted it into categories, and it called as Article Directory, because of it as long someone wants to be Contributor in Article Directory and submit your content there you need to follow their submission guidelines.
Before Google releasing Panda Algorithm in February 2011, Articles Submission to the Article Directory is very popular in webmasters circles. Article Submission not only helps them a lot to build backlinks which most of Article Directories let the users copy and re-post them somewhere with Links in the content body stay intact, and Article Submission also can boost their traffics by a lot. When Panda Algorithm release Article Submission becomes decline and not many webmasters still do that.
The Article Directory becomes more declining than before because of on April 24, 2012; Google releases their newest algorithm known as Google Penguin which targeting websites which make backlinks from Article Directory.
To gain favour from the search engine use article directory, you need to create unique content, write the content at least contain about 400 or more words, keywords density in the articles body no more than 3 to 4%. The most important are do not write articles to promote some products, affiliate links or business excessively and tag the affiliate links with rel nofollow, Articles which write in case study type will be much better than review articles, well research and original article will give more extra point for you.