Article directories are sites where you can post and article or a link to an article that is on another site. There is usually no limit as to how many articles you can submit. With links however most directories have limits which can be per day, per week, per month and so on.
HubPages's articles must be original so they cannot be published on any other site including directories, but links can. Think of a directory like a place where you can post a link that will drive traffic to your hubs and depending on the rank of these directories this can prove to be beneficial. Word of caution; posting too many links can make Google look at you as a spammer, albeit probably temporarily.
Some of the best directories are according to rank with 1 being the best:
1. knol.google.com (why I am not surprised by this one?)
4. HubPages (yes...that's us)
There are many more , but these are the top 10. Keep in mind that most of these do not allow links only although articles with some links are mostly welcome.
list can be found here:http://www.vretoolbar.com/articles/directories.php