Can somebody define Public relations

After working in Public Relations Consultancies, trying to understand the intrincities of communications, planning, execuating plans and discussing the true defination of Public Relations, have realised that everybody is defining PR the way they think of defining it. I personnaly tried to search the revelant defination from the web, and below is the list of my findings.

Public relations (PR) is the practice of managing the flow of information between an organization and its publics.
http://en.wikipedia.org/wiki/Public_relations

Public Relations: Public Relations is a form of communication primarily directed towards gaining public understanding and acceptance. Public relations usually deals with issues rather than products or services, and are used to build goodwill with public or employeess. Examples of public relations are employee training, support of charitable events, or a news release about some positive community participation.
http://www.learnthat.com/define/view.asp?id=52

Public relations: The art or science of establishing and promoting a favorable relationship with the public.
http://www.answers.com/topic/public-relations?cat=biz-fin

Public relations include ongoing activities to ensure the overall company has a strong public image. Public relations activities include helping the public to understand the company and its products. Often, public relations are conducted through the media that is, newspapers, television, magazines, etc. As noted above, public relations is often considered as one of the primary activities included in promotions.
http://www.managementhelp.org/ad_prmot/defntion.htm

Public Relations practice is the planned and sustained effort to establish and maintain goodwill and mutual understanding between an organization and its publics.
http://www.buildingbrands.com/definitions/07_public_relations.php

WHAT IS PUBLIC RELATIONS??? And how does it differ from ADVERTISING? It is really important to understand the difference…Public relations is the opposite of advertising (while it is one very important mode of publicity). In advertising, you pay to have your message placed in a newspaper, TV or radio spot. In public relations, the article that features your company is not paid for. The reporter, whether broadcast or print, writes about or films your company as a result of information he or she received and researched.
Publicity is more effective than advertising, for several reasons. First, publicity is far more cost-effective than advertising. Even if it is not free, your only expenses are generally phone calls and mailings to the media. Second, publicity has greater longevity than advertising. An article about your business will be remembered far longer than an ad.
Publicity also reaches a far wider audience than advertising generally does. Sometimes, your story might even be picked up by the national media, spreading the word about your business all over the country.

Finally, and most important, publicity has greater credibility with the public than does advertising. Readers feel that if an objective third party-a magazine, newspaper or radio reporter-is featuring your company, you must be doing something worthwhile.

The more you brainstorm the more differences you can list down, the list continues…

As your business grows, it naturally becomes a more prominent element in your community and your industry. PR is an excellent tool because it gives you exposure you don't have to pay for directly. Good PR rarely happens without effort. Getting good publicity usually requires careful planning, persistent effort, and, often, spending money for press release mailings, copywriters and PR consultants.
The good news is, as the founder of a growing company, you're in a prime position to be listened to by consumers and the news media. All you have to do is let others know you exist and that you are an expert source of information or advice about your industry. Being regarded as an industry expert can do wonders for your business.