International humorist Stephen Leacock defined advertising as: "the science of arresting the human intelligence long enough to get money from it." But the textbook definition of advertising is: "a form of persuasion that informs people about the goods and services they can purchase."
Advertising is very different from public relations. One key difference is that you always pay for the space and time of an advertisement (or commercial, which is an insert appearing on radio, television, or the Internet). By contrast, editorial coverage generated through public relations is not paid for by the organization issuing the news release. The media will pick up and publish the story because they consider it newsworthy, not as a paid advertisement.
Another crucial difference is that, in advertising, you have virtually full control over the message. Because you are paying for advertising, the ad or commercial runs your exact text (called copy), provided the copy complies with generally acceptable standards for advertising. In the case of public relations, the media outlet you are targeting is under no obligation to run the story in any form. If a media outlet does decide to run the story, an editor will generally rewrite the news release, or use pertinent information from the news release to create the news. (For instance, your news release might be used as part of a larger story on players in your industry or profession.) In addition, you have no control over when the release or news will run. All decisions are made by the editor.
As you can see, public relations is a cost-effective way of getting your story out. Taking the trouble to write effective news releases and to build a relationship with the relevant media will, in time, pay dividends in the form of exposure and prestige. Best of all, public relations probably costs less than a single advertisement.