Although the terms search engine and subject directory are used interchangeably they do not mean the same thing. A Web directory organizes Web sites by their subject, and is usually maintained by humans instead of software - this makes them more dynamic. The searcher looks at sites organized in a series of categories and menus. Web directories are usually much smaller than search engines' databases, since the sites are looked at by human eyes instead of by spiders. This sets up a limit at the number of pages that can be scanned per day while having a higher level of quality which is particularly useful for business web directory. There are two ways for sites to be included in a Web directory's listings; either the site owner can submit the site to the Web directory, or the directory's editor comes across the site and includes it by himself. Including your blog in a free web directory is a good way to increase traffic and PR. Examples are the very famous DMOZ, the yahoo directory, and BOTW. Almost everyone has a blog these days so i hope this post was of use to someone ..
The mobile web directories that are sprouting up these days are young so they lack in features but they are buding anyways.
BTW, i'm on the lookout for a co-blogger here so if you're interested do let me know.