The thing that's so wonderful about the web is that it's so easy to put up a website. And the thing that's so frustrating about the web is that it can be hard to get people to come look at your stuff.
Directories provide a structure for the web, and search engines suck the web into a soup.
One is like the table of contents in front of a book and the other is like the index in the back of the book, but neither is exactly like their print counterpart. Further, directories rarely address the issues of cross-referencing, offering a single hierarchy and listing a site only once. And it can be difficult to get a site listed at Yahoo. These are the thorny issues that web entrepreneurs and technologists spend a lot of time thinking about. There have been a wide variety of attempts to tweak and recombine the search engine and directory formula, including Alexa, About, Ask, Deja, Google, GoTo, Netscape, Third Voice, to name just a few.