Directory advertising is where you place an advert (either FREE or paid for) in local advertising directories such as the Yellow Pages or the Thomson Local, or online on the Internet using Yell.com, Freeindex or something similar.
Given that most directories will give you a free listing, it makes sense to take them up on the offer. Be advised though that as soon as you're listed in one, you'll get calls from others trying to sign you up, often for more expensive paid for adverts. Whether you opt for a paid advert is up to you, but be sure to read the small print and understand the costs involved before signing up.
With regards to online directories, it's a good idea to submit your details to as many free ones as possible. If you have a website, many directories allow you to include your website address which may help your search engine rankings. Visitors clicking on the link will also be redirected to your website which is a bonus.
Below is a list of directories all offering FREE listings: