The job of a public relations company is to be the go between for a business and the press / public. They often write and publish press releases to go in the local papers, they may arrange for radio adverts or interviews and will often help a business make contacts within the media industry.
Some PR companies will write news articles for a business that can then either be published on their website, sent out to their customers in a newsletter or for both purposes.
Social media has added another level to PR. It is a great tool for public relations companies or individuals to use as it allows them to reach a large audience instantly. As more and more people use social media and the technology advances, it is vital for a public relationship manager to fully understand all of these and know how best to implement them for that particular business.