Directories are comprised of staff or volunteers who review web site submissions, typically consisting of individuals or a team of reviewers. Submissions are accepted or rejected; usually directories request more information about the site such as a description and/or keywords. From my own experience, I've found the directory listings tend to be more stable.

Open Directory Project (ODP) is an open source directory used by the following search engines for directory listings:

AOL, Lycos, Netscape and HotBot. Here is the official list from ODP of sites that use their directory:

Sites Using ODP

The search engine ODP directory listings are refreshed at varying intervals. You need only submit to one of the search engines to submit to ODP as the submissions all go to ODP. Do not submit mulitple times as it will be seen as spam. Also, a safe way to submit is to choose two possible categories for submission. Contact them after 2 weeks by email if you are not listed yet. They do not send out acceptance emails.

ODP -- How To Add A Site

ODP Submission Tips

Yahoo! is the most popular directory at this time. Be sure to follow their rules closely. They want good content, a concise description without hype and for you to find the best category to submit to. Take the extra time to click through the categories and be as specific as you can. Only after submitting three times over a period of two to four weeks between each submission should you then contact Yahoo! to ask for information about improving your web site. Follow their suggestions and then re-submit. The contact email for support at Yahoo! is url-support@yahoo-inc.com

A great place for advice about submitting to Yahoo! is Search Engine Forums:
Yahoo! Forum.

How engines and directories feed the search engines:
Inktomi engine powers many search engines, sometimes for secondary results when the search results are not found in main search engine results.

Re-submission to search engines:
As a general rule wait two to four weeks before re-submission to directories and search engines.