A PR agency is a Public Relations agency and their role is to analyse your business and find out about the products and services you offer. They will look at the positive messages you are portraying and then use these to promote your business within the media industry.
Although PR companies do not come cheap (you can expect to pay a few hundred pounds to a couple of thousand pounds) their influence and advice can be vital to a business looking to promote themselves of a particular product / service.
Many businesses use a PR company to launch a new product as they often have great contacts within the media and can quickly get stories and press releases out to the public.
You may want to search for a PR company on the internet, but be sure to do your research to find out how good they are before signing up with them.
Many PR companies will be more than happy to give you case studies of work they have done for other clients and discuss how it helped them.