That is a truly great question. We are all "public relations/relators" in a way...anytime you deal with the public you are relating...sounds very basic and sometimes it is. However, when you do public relations for a company or agency, you you are representing the agency and managing the information that is put out to the public. Nowadays, managing a company or agencies social media presense and information in a big form of public relations. The person that works for a company/agency and posts on their Facebook page is probably the best example of a public relations person. Hope this helps.
The below is pasted in from www.howstuffworks.com/businesscommunications:
Careers in Public Relations
Public relations professionals are natural and effective communicators -- skilled writers, journalists, public speakers and cold callers who are media-savvy and well-versed in pop culture and current events [source: The Princeton Review].
There are a variety of careers in public relations because the nature of the work is so diverse. That said, the broad scope of public relations jobs can be narrowed down into three general categories:
The bread-and-butter of public relations is generating publicity. Publicity is a free and favorable mention of your client in a magazine, a positive review of your client's product in a newspaper column, or a recommended link to your client's Web site on a popular blog.
All careers in public relations include a certain amount of publicity work, but the real heavy lifting belongs to publicists. Publicists spend their days writing press releases to targeted journalists and publications. They handle all press inquiries regarding their clients and they prepare detailed press packets to hand out at interviews, press conferences and trade shows.
Public relations are in short the people within a company who handle external media. When the company releases news about changes in leadership or finances or things like that, they're heavily involved to make sure it is portrayed well and can also be involved if there's a controversy about the company in the media to make sure the company's side of things is heard.
I agree with the answers above. What I would add is that PR professionals really both Promote and Protect a company.
So - when a business has positive news, a new product or something great to say to their target market PR is core to communicating the key messages to the media in order that they write about the company in a good light. PR is an exceptional brand awareness tool.
Protecting the business is equally important - the PR team will work with senior management to ensure the brand and the company are minimally impacted when there is more negative news that needs to be communicated.
Third party endorsement is a key term when describing PR - the role is to work with third parties (the media, customers, partners, analysts) in order that they promote your brand and business for you - which then has more impact and authenticity.