Sooner or later you realize that you need to do something with your website promotion to get more visitors and generate more sales. You know that search engines bring huge traffic, and you start doing SEO (Search Engine Optimization) work for your website. You know that quality text content and correct meta tags are very important, and more important are links which lead to your website from other websites.
Submission to web directories is one of the main ways to get links for your website.

Web directory is a web site that includes categories and sub-categories where categorized websites listings are posted. When you add website to online directory, a link to your website is added. As you know, Google Page Rank (further PR for short) and link popularity are important factors of high search engines rankings. When your link is placed on a page with good PR is a plus to your link popularity, because your website get piece of the pages PR. The more links your web site has the higher your rankings in search engines and the links must be natural i.e. relevant and one-way.
Few years ago for high rankings in search engines it was enough just to exchange links with other websites. Nowadays Google recognizes such schemes and considers these links not natural, and as result discounts them in website popularity and PR calculation. Reciprocal link exchange can still be used as way to get visitors to your website if you exchange links with good high traffic websites, but for building ranking one-way natural links are needed.

The links must be SEO-friendly: static, direct, and without "nofollow" attribute; they must be on a page that has some PR, surrounded by text content related to your website theme and not more than 30 other websites links. More links on a page will devaluate piece of PR your website gets from the page.

Web directory that offers such links is called SEO friendly web directory and is really worth even paying some small amount for submission because your link will be found and indexed by search engines in just a couple of days and it will be a "plus" to your website link popularity.
Usually web directory listing consists of linked title, URL, and description, and also can display website home page thumbnail, Page Rank, detailed description, reviews by visitors, and other. Listings can be free, free with reciprocal, regular paid, and paid premium.

Free regular listing's plus that you pay nothing for inclusion, but this type of listings doesn't guarantee permanent place for your link because newly submitted links will move it to the further pages with lower PR.

Option of free with reciprocal link listing you can select in a paid directory if you don't want to pay but want to add your website. Some web directories consider this type of listings as a kind of premium, and give benefits of placement to your link until you keep their reciprocal link alive.

Regular paid listing you may pay for inclusion to a really good web directory, which is SEO friendly and will really give positive results to your website promotion.

Premium paid listing is always placed above regular listings and can be added to top categories pages with good PR. Such type of listing guarantees permanent top place until payment expiration, which can be monthly, quarterly, or yearly. Often premium listing allows deep links submission, i.e. link to internal pages or sub-directories of your website, that is good for your website promotion.

How to find good web directories? Of course you can use search engines but better is to use categorized list of reviewed and rated web directories at special websites like http://www.criticdirectory.com, http://www.directorycritic.com, http://www.addurl.nu/, http://info.vilesilencer.com/ where the directories are reviewed by hundreds of webmasters so you dont waste your time on submission to useless directories.

As you will see from the lists, there are general and niche web directories. Always start submission from niche web directories related to your website theme, because links from relevant resources will be more useful for your website, and then go to general web directories.