There are several different types of link directories in existence today.
First, there is the “Open Directory Project,” otherwise known as the OPD. It is manned by real people who will check the relevance of the web sites that are submitted. Only those that are deemed relevant will be included so that users will find only useful information, not spam or junk.
Secondly, there are free directories. It is in your best interest to submit your web site to as many of these different directories as possible because it will give your web site credibility. However, it will take months for your web site to appear in free directories because there are so many web sites submitted to these directories that editors have to go through each day. Plus, there is no guarantee that your web site will even be listed in these directories if another web site that is already there has the same content as yours.
Third, there are also paid directories. Usually, if you submit a web site to these directories it will be accepted. However, the fee that you have to pay to get your web site listed can be very expensive. For instance, it will cost you $300 per web site that you want to get listed in the Yahoo directories.
Overall, the time and effort that you spend submitting to these directories is worthwhile because people use these as an online library. This means that you will surely get visitors to your web site.