The Open Directory Project, commonly called ODP or DMOZ, is the largest directory online and is administered by Netscape (an AOL Time Warner Company), although it is made up mostly of volunteer editors. The ODP has been set up as an open source directory that anyone can freely use, so long as they follow the license agreement. This fact makes ODP very popular among smaller websites, who wouldn't have the funds to pay for a directory otherwise.
The use of ODP data isn't limited to smaller sites however. The directory was used as a primary result source for AOL Search and Netscape Search for a few years. Additionally it is currently used by Google for their directory and it is spidered by nearly every search engine.
If you get a listing in the ODP you won't notice traffic from it right away, the traffic coming from the actual directory website is slim, instead within two weeks to a month you should be getting traffic from AOL, Netscape, Google, and the others.
On the ODP's front page they boast over 40,000 editors. This is not an entirely accurate number because it includes former and inactive editors. The truth is the actual amount of editors who are active and reviewing sites are relatively few, because of this it can take a long time for your submission to get reviewed. If your category does not have an editor it is not uncommon to wait up to sixth months or more for a review. ODP guidelines state that you should submit again after 3 weeks and if your site is still not added that you should write an editor, either of the category in question or of a parent category, and to ask for a review. However most editors will ignore your email and if your submit too much, even if you wait 3 weeks between submissions, a poorly trained editor may take your submissions for spam and delete all of them. To help get your submission added I would suggest reading the editing guidelines and try to write an acceptable objective description to make it easier on the editor who would have to add it. If the editor can simply choose to add a site without having to make revisions to the submissions it is much easier to get in.
Poorly trained editors are not the only problem at ODP, because it is run almost exclusively by volunteers it has evolved into an oligarchy, which is typical of volunteer organizations. Also the iron law of oligarchy, a sociological term for the tendency of organizations to be dominated by a small self-perpetuating elite, has taken full hold as well. As such the ODP is run more like a club house or a fraternity than anything else. Guidelines are not uniformly enforced and privileges are not always granted based on skill. As such it is not uncommon to find the quality of editing throughout the ODP to be highly variable. The ODP suggests that if you are submitting to a category without an editor that you should consider applying to be an editor in order to help out. If you decide to do this you should be aware of how things in there are run, just so you know what you're getting into.
The ODP also has no pay method of inclusion, so you have no way to speed up the submission process. If you are forced to wait you are forced to wait no matter how much money you have or you don't. This can be a good thing, it puts you on equal footing with the big corporations, but on the other hand if you've been waiting 6 months you probably would have been better off if you paid for it at the beginning.
For ranking its search results ODP uses standard keyword weighting. The more keywords found in your title, description, URL, and category name the higher you will be listed. They also have what are known as "cool sites," these are basically editor's picks and are listed at the top of the category in which they are found. They are also differentiated with a bold listing and a star icon next to their title. Not all categories have "cool sites," and not every category needs one. The title is reserved for sites that stand out above the crowd. Within the categories sites are ranked Alphabetically and, unlike Yahoo, DMOZ ignores numbers or punctuation in ranking alphabetically. Instead they simply rank the listing based on the first letter in the title.