When somebody says, "I want a press release," they don't usually want one. What they really want is to get something in the newspaper, on TV, or on the radio. "Press release" is just the phrase they're familiar with. They want to send some kind of document to the media.
The real question is, "What do you want people to do?"
Imagine that all your local newspapers, radio stations and TV newscasts ran whatever you wanted. The entire audience you targeted gets your message and paid complete attention to it. OK. Now, what are all those people supposed to DO?
With marketing and advertising, this question is simple. The point is to persuade people to buy Hondas, Nikes or iPods, and you can track how effective your advertising is by trying different things in similar markets and seeing how the cash registers ring.
Public relations is different. You're not selling widgets. You're informing the public -- or selling ideas. It's hard to tell what tools and techniques are working and which are duds. There are no sales numbers to run through Excel.
That doesn't mean anything goes.
Say you're the local sheriff and want people to hang up their cell phones and drive, because a new state law makes it illegal. One press release in the paper, or one appearance on a talk radio show, won't do the job. Not everyone listens to talk radio, and it's a good bet that the majority of the population wasn't tuned in precisely when the piece aired.
Instead of a single press release, it might take more. It make take an oped (guest column) persuading people to hang up and drive, plus public service announcements on television and radio. A real campaign won't take a day or two. It takes weeks, or months, to persuade people and change habits.
A single press release might work to publicize something simple and short-term. A good example is announcing a meeting.
For anything tougher, it takes more than one piece of paper. It takes a sustained campaign across all the different mass media -- radio, newspapers, television, blogs -- to make sure your whole audience (1) hears the message and (2) is persuaded by it.