Great explanation...

If the circus is coming to town and you paint a sign saying "Circus Coming to the Fairground Saturday," that's advertising. If you put the sign on the back of an elephant and walk it into town, that's promotion. If the elephant walks through the mayor's flower bed, that's publicity. And if you get the mayor to laugh about it, that's public relations. If the town's citizens go the circus, you show them the many entertainment booths, explain how much fun they'll have spending money at the booths, answer their questions and ultimately, they spend a lot at the circus, that's sales. - Unknown