Comms, PR and Marketing: is there a difference?

What's the difference between public relations, marketing and communications Depending upon the topic, it seems that people define the role of public relations practitioner, corporate communicator, and marketing fairly loosely. What exactly is the difference between the three distinct professions?

This graphic tries to separate them by indicating specific "benefits" of working in marketing communications (like travelling on business, having access to Super Bowl tickets) and then presenting the proportional odds of that benefit being available to one or all of the professions.

[tags] marketing, communicator, corporate communications, public relations, schwag [/tags]