To begin, it would be nice to have an explanation of what a directory and what a search engine.

The Directory :

A directory works like a telephone directory except that it is a website. The reference directory of websites ranked by category. When a site is a directory is a human being who decides which category the site will be placed.

The search engine :

The search engine is a robot that visits the website on a regular and automatic and reference pages on a specific algorithm (and secret). In general, you need not enter in a search engine finds your site by itself. For more details on how the search engines, see the article ''Comment fonctionnent les moteurs de recherche ?'' available on this site.

Now, why is it inadvisable to use the directories ?

Because Google and the other search engines recognize the directories when they visit and they do not like them at all. Bad directories are often associated with spam indexing. The directories are usually real ''links farms''.

By using such directories, you may do ''blacklisted'' (putting on a blacklist) the various search engines and in particular by Google is the most important among all. If Google (or other search engines) will place on its blacklist, you can say goodbye to traffic in your online store.

For against, there are some directories that are ''respectable'' and where it is desirable to register because Google (and other search engines) and visit the reference sites there. Among these fine directories, there are Dmoz and Yahoo.com.

About Yahoo.com :

Yahoo.com is a directory only in its infancy, it was only later that it became a search engine. Now, besides being an excellent book, it is the second search engine most used by Internet users (after Google).