Marketing is the act of selling a product or service and all the business activities that are entailed. Marketers determine the process involved in bringing products or services together with potential buyers and investors. Generally speaking, if you are a marketer, the more products or services you sell, the more successful you are. Sometimes marketers don't care how they do that and resort to nasty practices like telemarketing and spam to help sell their stuff. This practice is akin to yelling out the window of a moving car.
Advertising is the art of creatively promoting a product or service. People in this occupation formulate a compelling message in a non-personal manner; strategically outlining unique benefits using a combination of various traditional and new media. There are two ends of the spectrum for agencies: 1) those in it for the money, who will put out any message as long as the client approves it, and 2) agencies who knock themselves out over creative excellence. The more creative the work is, the easier it will cut through the clutter and reach the minds they are targeting. They figure if they make a name for quality work, the money will follow. Silly creative people.
Marketing, advertising and public relations are three different, valuable disciplines and come from different focuses and perspectives. Working together, they can help build a successful business effort. But clients should not be fooled into thinking that one skill set can do the job of the other.