I'm impressed the “What is the difference between public relations and advertising?” During my work experience as a marketing  person ,I've done a lot of PR and AD job. I think everyone agrees that the use of both advertising and public relations is the most effective way to create awareness and promote you and your business. Few people, however, can distinguish between these two methods of marketing.

Advertising provides a direct line of communication to your existing and prospective customers about your product or service.

Public Relations allows you to tell your story indirectly through third-party outlets, primarily the media. PR has the credibility which advertising does not have today. If you don't know anything about a new product or service, you are likely to believe everything you read about the subject, especially if it comes from a credible source. Public Relations is free and comes from what people are saying about you and your company.

PR builds awareness of your product or service which is critical for startups and entrepreneurs, and advertising supports the brand when it gets known.
PR builds credibility, Advertising builds visibility. What’s cool about getting PR is because it’s written by a journalist, there’s instant credibility and other people become more interested. If you’re a small business, it can turn you from a nobody into a somebody who can compete against a bigger business.

Are the lines sometimes blurred between Public Relations and Advertising? Absolutely. As advertising agencies become more clever, ads sometimes seem like third-party endorsements. When infomercials were first introduced, many consumers thought they were watching a talk show (a venue for public relations) when they were indeed watching a paid advertisement. And when a radio announcer does a live tag at the end of a recorded ad it may sound like an impartial testimonial, when again, it is paid advertising.