There are a lot of different ways to promote your business. Traditionally, the most common model has been to pay for advertising in some popular venue, purchasing ad space on a television program, radio show, or in a magazine. There are also real world advertising models that follow the same pattern, buying banner space at a large function, or paying to have your ad placed on busses and taxis. Even in the new world of internet marketing, the basic method is to pay for clicks, from a search engine, from a media outlet, or from a social networking site. In general, marketing means paying money just to get your message heard.
Craigslist is a different arrangement altogether. Based off of traditional classified ads which have run in newspapers for decades, Craigslist allows users to post a listing offering or asking for almost anything they can want. The difference here is that Craigslist is absolutely free.
The fact that posting an ad is free is more than just a great way to save money getting your message out there. It is also a way to inexpensively test ad messages. It is possible to set up listings so that they have a link, pointing back at a landing page that you control on a website you own. This can be outfitted with a tracking device, letting you know exactly how many people clicked through, thus telling you how effective the ad was.
This is a much better setup than with traditional media, where you have to spend money just to see if people like the message. Even then tracking results is often difficult. If you want to make these efforts even more effective, consider employing a Craigslist marketing company that will have the tools to set up split testing ads and find out exactly what works best for your business.