What are the differences between traditional PR and modern PR?

Actually, there are many similarities, but the biggest difference is that traditional Public Relations relied on face-to-face communication, usually by creating a big event and inviting many people to it. In the early 1920s, when the great PR man Edward Bernays wanted to encourage women to smoke (back then, it was not well known that smoking was bad for one's health), he created a parade and showed women smoking as a sign they were now emancipated-- they had gotten the vote and now they could smoke in public if they wanted to. He called the cigarettes "torches of freedom" and many spectators who saw the parade also saw the attractive women smoking cigarettes during the event.

Today, modern public relations makes use of media, far more than was possible in the old days. Publicists use social media (such as Facebook or Twitter) or television or other forms of mass communication, making it easier to get the message out and attract a large audience, with or without getting people to go somewhere to see the client or the product. Political candidates often rely on PR to shape their image, and while face-to-face events are still important, so is the ability to use the mass media to promote the candidate and give potential voters a positive impression of him or her.