A PR agency is a public relationships Agency that will promote your business through different types of media both online and offline if that is what is required. Usually once you have spoken to an PR agent, they will come and see your business in action. They will find out a bit of background information about the company and get to know what services or products you offer. They will be looking to find out information that they can use in media releases to newspapers, social media sites and other publications to increase your brand awareness and attract new business.
To find a good PR agency you should try and get some testimonials off clients that they have worked with in the past. This will give you an insight in to the kind of results you can expect to get. If you do hire a PR agency you need to be prepared to work closely with them and dedicate them some time as they are only as good as the information they have access to or are given.