From Wikipedia — the definition of PR:

Public relations (PR) is the practice of managing the flow of information between an organization and its publics. Public relations – often referred to as PR – gains an organization or individual exposure to their audiences using topics of public interest and news items that do not require direct payment. Because public relations places exposure in credible third-party outlets, it offers a third-party legitimacy that advertising does not have.

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It continues:

The Public Relations Society of America (PRSA) claimed in 1988: “Public relations helps an organization and its publics adapt mutually to each other.” According to the PRSA, the essential functions of public relations include research, planning, communications dialogue and evaluation.

And FYI, if the word “publics” makes you go “whaa?” It’s okay. I hate that word, too.