Here is how the Chartered Institute of Public Relations describes public relations:
Public relations is the discipline which looks after reputation, with the aim of earning understanding and support and influencing opinion and behaviour. It is the planned and sustained effort to establish and maintain goodwill and mutual understanding between an organisation and its publics.
Here is what one of the world's largest PR firms, Burson Marseller, claims to do on behalf of its clients...
Burson-Marsteller is a leading global public relations and communications firm. Our strategic insights and innovative programming build and sustain strong corporate and brand reputations. We provide our clients with counsel and program development across the spectrum of public relations, public affairs, digital media, advertising, and other communications services. Our clients are global companies, industry associations, professional services firms, governments, and other large organizations.
Here is how B-M does it: Facebook exposed in Google smear campaign