Web directories are web sites powered by a specific script. Their whole purpose of existence is to accept or deny links from various suggestions. Anyone can suggest a link but most of the times links are suggested by a webmaster or by a site administrator, as they are seeking to submit a new or older site for inclusion. Submitted sites are usually reviewed by a human editor that works for the company that owns the web directory. This review is needed to ensure that only quality content gets posted in that directory.
WHAT TYPES OF DIRECTORIES ARE THERE?
A typical web directory would consist of many main categories, each one being divided into somehow smaller and more niche targeted categories. Usually most sites are listed in the smaller categories but some directories accept submissions to the larger categories as well. Such a directory can become very big in time as there are more and more sites added to each category.
A certain type for directories is the niche directory. This can be also known as a specialty directory, a directory that specializes to one category of themes, services or products. Such a directory is very useful and of course it is less overwhelming than the vast content of a general directory.
Directories that ask for money when someone suggests a web site (a.k.a. Pay A fee web directories) are directories you should usually stay away from. They normally ask for a review fee but most of the times there is no guarantee that your site will be approved and there is no refund policy unless stated. There some exceptions to this rule. Some directories that ask for money will automatically list your website upon payment, with no review from a human editor. This will translate in a quick link.
Reciprocal directories are not to far away from the paid link directories. They involve a mutual linkage between two websites in order to ensure traffic and link juice being exchanged. The site that is submitted to the web directory must place a reciprocal link on it before inclusion or approval. As with the payment in paid directories, a reciprocal will also not guarantee the inclusion. You should beware of reciprocal link directories because the whole linking scheme is set up so the directory always gets most link juice. After some time these directories get very big and have lots of visitors because all of the inbound links and they eventually turn into paid directories.
Free or seo friendly directories are the best kind. Although this directories also might offer other link types like reciprocal or paid they also offer a free link. This is what makes a directory be of this type. The free link is usually a do-follow link that by definition doesn't cost anything. Your website will get listed in a related category. This type of one-way links is the best type of links.
Submitting a site to web directories is the best idea of promoting any site for building some back links to start with. This should be the first step of every link building promotion.