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This is a loaded yet good question. Marketing is communicating the value of a product to a customer (writing and creative), pr is the managing of information between the organization and the public (writing) and advertising is the act of producing advertisements (more extensive than marketing but similar) for a product or service (creative/art).
With social media we are seeing the lines of marketing and PR blur but they are still both equal and important fields. Advertising is important too as it is what gets the message across to us - think billboards, magazine ads, etc.