Robert Wynne, Forbes contributor, defines what Public Relations Agencies do in his article "What Does A Public Relations Agency Do?" He starts by outlining what what PR firms don't do.
They don't buy ads, or write stories for reporters. They don't sell ad space or come up with ad campaigns to sell products to consumers.
He goes on to describe what PR firms actually do. A PR firm is something distinct and separate from an ad agency. PR firms "promote companies or individuals via editorial coverage. This is known as “earned” or “free” media — stories appearing on websites, newspapers, magazines and TV programs — as compared to “paid media” or advertisements."
The goals of Ad agencies and PR agencies are the same. They're both attempting to paint the best possible light. They're attempting to create an image of trustworthiness and credibility to whomever has retained their services.
So, what's the difference? Well, advertising is generally looked at as a sort of self-promotion. The client pays an ad company to create a campaign that promotes them. PR on the other hand is less about delivering a message to the audience, and more about maintaining a dialog between a client and their audience. It's about understanding the client, whether it's a company or an individual, well enough to see how to best represent them in the media. It's helping clients put their best foot forward so to speak.
The role of the PR agency or professional can be thought of as that of a mediator between the client and their audience. Can be a helpful partner in implementing ways for a company or indivual to "talk" to their audience. They promote the positive image of clients through media stories, and because they are not direct employees of their clients they are supposed to be able to give an outsiders perspective with regards to what's working with their client's image and what isn't. This seems like it would really come down to how scrupulous the company really is.
According to Wynne many PR professionals are former journalists who know how to pitch stories to editors. If the story sounds good then the editor may put a journalist on it to investigate and report back in as unbiased way as they can. This removes the money trail. At this point it's on the journalist to report whether or not the company is on the up-and-up no matter how good the hors d'oeuvres are at the PR event.
PR professionals also write speeches to make sure the CEOs of client companies are delivering a message the is in line with the image they are trying to promote. They are paid to be aware of topics that may be of a sensitive nature to an audience and avoid them. In short they are paid to know who to talk to and how to talk to them on behalf of a client. Many times this involves knowing which medium to best reach an audience through.
Wynne describes PR firms as an investment for companies. PR firms promote an active relationship between clients and their audiences. A client describes what image they want to portray and where they want it seen and it's up to PR firms to make that happen.
Wynne, Robert. "What Does A Public Relations Agency Do?" Forbes. N.p., 10 Apr. 2013. Web. 12 Sept. 2014.